structural geology 234.txt

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Structural GeologyIntroductionStructural geology is the study of the features formed by geological processes.Features include faults, folds and dipping strata. Geologists can work out the order of events and see which events are related by taking fairly simple measurements and using simple methods.Measurements and TechniquesThe most obvious thing to do when trying to decipher the structural history of aformation is to describe it. One way of doing this is to measure the dip and strike. The dip is the amount a bed of rock is tipped from the horizontal. The strike is the direction which is ninety degrees from the dip, i.e.. Along the horizontal line on the bed.The strike can be in two directions, hence the dip could be in one of two directions also. There is a convention for the strike to be the in the direction you are facing if the rocks are dipping to your right. Some geologists prefer to measure the dip direction, rather than strike, as it is slightly simpler. However, all maps use dip and strike, not dip direction.This is complicated slightly by apparent dip. This is due to the fact that you are not always looking edge on (perpendicular) to the bed you are measuring. If you are looking at a bed at a slight angle, then you see the apparent dip. The true dip will be greater than the apparent dip, as it is the maximum amount of dip, so the apparent dip can appear to be anything from 0 to the maximum (true) dip.In this diagram, the dip is 30 with a strike north/south (0 /180 ), the dip directi

on is 270On a geological map, symbols are used for the dip and strike. The strike is represented by a bar, and the dip by a mark on the strike bar on the downdip direction with the dip written alongside, as shown on the map below left. A geologicalcross section can be drawn from the map showing the subsurface structure. Obviously, only features which can be seen on the surface can be represented. The cross-section below right is drawn using the values in the map alongside.A technique which is used often is to plot values of dip and dip direction on astereogram. A stereogram (or stereonet or hemispherical projection) is a way ofrepresenting 3 - dimensional directions on a2 - dimensional surface. The net isa projection from the point onto the equator.The points are placed all around the sphere representing 3D space. The points are projected down onto the equatorial plane on a line which meets up at the south

poleFoldsFolding of rocks is caused by the compression of rocks. This occurs slowly, overa long period of time. If this happened quickly, the rocks would break, and fault. This is due to the mechanical properties of rocks, namely it's plastic nature. If a rock is stretched slowly, then it will behave in a ductile fashion. If stretched quickly, the rock behaves in a brittle fashion.Nomenclature used when describing foldsFolds are classified by shape and the chronological order of rocks in them. Theshape of a fold is described by the angle between the limbs, which are given theterms: Gentle (120 ? 180 ), open (70 ? 120 ), close (30 ? 70 ), tight (5 ? 30 ) orisoclinal (0 ? 5 ).Hinge: Where curvature of the fold is at a maximum

Crest & Trough: Where fold surface reaches a minimum and maximum respectivelyLimb: Beds between two hingesAntiform & Synform: Convex upwards or convex downward folds respectivelyAnticline & Syncline: Older or younger beds at the core respectively. Can be used in conjunction with antiform and synform, i.e.. An antiformal syncline


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SedimentologySedimentary rocks are made by the accumulation of particles of older rocks, either as clasts (chunks of rocks) or as mineral grains, chemically or biogenicallyprecipitated. Clastic sedimentary rocks are principally classified on the basisof grain size and then further divided in terms of mineralogy. One of the most important things sedimentary rocks can tell us about is palaeoenvironments-ancient environments. This is done by looking at the sedimentary structures and the fossils contained within the rocks. They are also an important resource for oil, gas and coal. This article concentrates on clastic sedimentary rocks. The carbonate tutorial for more information on a chemically precipitated sedimentary rocks.ClassificationClassification of sedimentary rocks is based principally on grain size. Grain size is measured in millimetres and is the approximate diameter of a single grain.There are several aids for estimating grain size in the field as well as more sophisticated aids when using thin secitons of sedimentary rocks under a microscope. The table below gives the grain sizes and names of the common sedimentary rocks.NB: A conglomerate has rounded clasts, a breccia has angular clasts.Classification of clastic sedimentary rocks based on grain size.Diameter (mm) - Sediment Name-Rock Type> 256 - Boulder-Rudaceous-Conglomerate or Breccia

Between 256 and 64 - CobbleBetween 2 and 64 - PebbleBetween 2 and 0.625 - Sand-Arenaceous SandstoneBetween 0.625 and 0.0039 - Silt-Argillaceous siltstoneTexturesTextures in sedimentary rocks depend on the type of grains making up the rock.Roundness-the degree of rounding of a grain. Not to be confused with sphericity.Grains can be angular to well rounded. A well rounded grain has generally traveled further before deposition.Sphericity-degree to which grain is a perfect sphere. Does NOT depend on roundness.Sorting-the amount of different sized grains in a rock. Ranges from very poor towell sorted.

Matrix or cement-the finer grains in a rock (matrix) or a chemical precipitate (cement) holding the rock together. Common cements are calcite or quartz.Competence-the toughness of a rock.Other properties of a sedimentary rock are porosity and permeability. The ability to store fluid (e. g. Oil, gas or water) is the porosity. The porosity is expressed as a percentage and depends on the amount of pore space in the rock. The ability to allow a fluid to pass through a rock is the permeability. Fluid can pass through using cracks, fissures or space between grains. A high porosity rockcan have a low permeability if the pore space does not connect in three dimensionsStructuresThe structures in a rock tell us a great deal about the palaeoenvironment. Thisis where one of the great sayings in geology comes in use:

The present is the key to the past - the law of uniformatarianismThis essentially means if we can understand what processes occur today, for example, the forming of ripples in a tidal mud, then these principles can be appliedto the geological record. Below are some examples of sedimentary structures andwhat formed them.A way-up structure tells us which way up the bed was originally deposited. Graded bedding usually occurs with the coarse grains at the bottom. If you find somegraded bedding with coarse grains at the top, then the bed has probably been tectonically turned upside down (e. g. By folding).Cross Bedding (or stratification). The entire dune as around a metre in height.


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These cross beds were formed in a shallow fluvial environment, which can determined using the relatively poor sorting of the sandstone.Mudcracks formed from the drying out of mud and then preserved in the rock. Thescale on the left shows centimetres and inches.

RocksWhat Is a Rock?Everyone knows what a rock is, until you ask what it is exactly. After some thought and discussion, most people will agree that rocks are more or less hard solids, of natural origin, made of minerals. But all of those criteria have exceptions.Rocks Are NaturalNot entirely. The longer humans stay on this planet, the more that concrete accumulates. Concrete is a mixture of sand and pebbles (aggregate) and a mineral glue (cement) of calcium silicate compounds. It is a synthetic conglomerate, and itacts just like the natural rock, turning up in riverbeds and on beaches. Some of it has entered the rock cycle to be discovered by future geologists. Brick, too, is an artificial rock in this case, an artificial form of massive slate.Another human product that closely resembles rock is slag, the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, such as in road building and concrete aggregate. It too has surely found its way into sedimentary rocks already.Rocks Are Made of Minerals

Many are not. Minerals are inorganic compounds with chemical formulas and mineral names, like quartz or pyrite (see What Is a Mineral? ). But what about coal? Coal is made of organic material, not minerals. The various types of stuff in coalare instead called macerals. Similarly, what about coquina, a rock made entirelyof seashells? Shells are made of mineral matter, but they aren'tnminerals any more than teeth are:Rocks Are HardNot necessarily. Some common rocks can be scratched with your fingernail: Shale,soapstone, gypsum rock, peat. Others may be soft in the ground, but they hardenonce they spend time in the air (and vice versa). And there is an imperceptiblegradation between consolidated rocks and unconsolidated sediments. Indeed, geologists name and map many formations that don'tnconsist of rock at all. This is why geologists refer to work with igneous and metamorphic rocks as hard-rock geolo

gy, opposed to sedimentary petrology.Rocks Are SolidMost are complete solid. Many rocks include water in their pore spaces. Many geodes hollow objects found in limestone country hold water inside them like coconuts. And the fine lava threads called Pele's hair, and the fine open meshwork ofexploded lava called reticulite, are barely solids.Then there's the matter of temperature. Mercury is a liquid metal at room temperature (and down to 40 below zero), and petroleum is a fluid unless it's asphalterupted into cold ocean water. And good old ice meets all the criteria of rockhood too, in permafrost and in glaciers.Rocks like these are not controversial, but they have their own category: Biogenic rocks. Perhaps concrete and slag could be added to that category too. Concrete would fit in with the others, being essentially sedimentary, but slag would pr

obably be a biogenic igneous rock.Finally we have the exception of obsidian. Obsidian is a rock glass, cooled so quickly that none of it has gathered into crystals. It is an undifferentiated mass of geological material, rather like slag but not as colorful. While obsidian has no minerals in it per se, it is unquestionably a rock.Types of RocksIgneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava.Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks.


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Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica.Size and HrdnessGrain Size: Coarse grains are visible to the naked eye (greater than about 0.1 millimeter), and the minerals can usually be identified using a magnifier; fine grains are smaller and usually cannot be identified with a magnifier.Hardness: Hardness (as measured with the Mohs scale) actually refers to mineralsrather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, hard rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6 ? 7 and up); soft rock does not scratch a steel knife but scratches fingernails (Mohs 3 ? 5.5); very soft rock does not scratch fingernails (Mohs 1 ? 2). Igneous rocks are usually hard.Origin of Igneous RocksIgneous comes from the Latin for fire, and all igneous rocks began as hot, fluid material. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.Igneous rocks form in three main places: Where lithospheric plates pull apart atmid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush. a liquid carrying a load of mineral cr

ystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field, and this article is only the barest outline.Igneous Rock TexturesThe three types of igneous rocks apart by their texture, starting with the sizeof the mineral grains. Extrusive rocks cool quickly (over periods of seconds tomonths) and have invisible or very small grains. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large aspebbles. Even a meter across. Because they solidified from a fluid state, igneo

us rocks tend to have a uniform texture, without layers, and the mineral grainsare packed together tightly. Think of the texture of a fruitcake, or the patternof bubbles in a piece of bread, as similar examples.In many igneous rocks, large mineral crystals float in a fine-grained groundmass.The large grains are called phenocrysts, and a rock with phenocrysts is called aporphyry; that is, it has a porphyritic texture. Phenocrysts are minerals thatsolidified earlier than the rest of the rock, and they are important clues to the rock's history. Some extrusive rocks have distinctive textures. Obsidian, formed when lava cools very quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles. Tuff is a rock made entirelyof volcanic ash, fallen from the air or avalanched down a volcano's sides. And pillow lava is a lumpy formation created by extruding lava underwater.Basalt, Granite and Other Igneous Rock Types

The main minerals in igneous rocks are hard, primary ones: Feldspar, quartz, amphiboles and pyroxenes (called dark minerals by geologists), and olivine along withthe softer mineral mica.The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.


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These two categories cover the great majority of igneous rocks. Ordinary people,even ordinary geologists, use the names freely (Stone dealers call any plutonicrock at all granite. ). But igneous petrologists use many more names. They generally talk about basaltic and granitic rocks among themselves and out in the field, because it takes lab work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow subsets of thesecategories.But a few of the less common igneous rock types can be recognized by non-specialists. For instance a dark-colored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. And there is a suite ofultramafic rocks with even more dark minerals and even less silica than basalt.Where Igneous Rocks Are FoundThe deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subductionzones, either in volcanic island arcs or along the edges of continents. However,continental magmas tend to be less basaltic and more granitic.The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granite eventually. In general, granitic rocks are less dense than basalticrocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle. The behavior and histories ofgranitic rock bodies are among geology's deepest and most intricate mysteries.

Introduction to Metamorphic PetrologyIntroductionMetamorphic petrology is the study of rocks which have been changed (metamorphosed) by heat and pressure. They are broadly categorized into regional and contact. Metamorphism is an extension of the process which forms sedimentary rocks fromsediment: Lithification. However, all types of rocks; igneous, sedimentary andmetamorphic, can all be metamorphosed. During metamorphism no melting takes place. All the chemical reactions which take place occur in the solid-state.Factors Controlling CharacteristicsThe characteristics of a metamorphic rock depend on the following factors:

Composition of parent rockTemperature and Pressure of metamorphismFluidTimeThe composition of the parent rock does not usually change during metamorphism (if it does it is then called metasomatism). The changes are the due to the minerals changing. A basalt which has around 50% of silica will produce a metabasaltwith 50% silica.Temperature and pressure affect the rock in terms of the mineral assemblage which is stable at the pressure and temperature obtained. The minerals stable at thepressure and temperatures that metamorphic rocks reach are simulated in a lab.This allows geologists to look at a mineral assemblage and give a (good) estimate of the pressure and temperature that the sample was exposed to. This gives tec

tonic information which is useful in other branches of geology.Fluid changes the chemical composition of the rock being metamorphosed and henceis called metasomatism. The addition of fluid can radically change the rock.Time has an important role as a rock which is heated to an extreme temperature for a short (years) period of time will not be altered too much. A rock heated for a longer period of time (millions of years) will show changes.ClassificationThe classification of metamorphic rocks is split into contact and regionally metamorphosed rocks. After this it is divided according to the amount of metamorphismthat has taken place and/or on the mineral content.


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Contact Metamorphism (based on mineral content)

Parent Rock-Metamorphic rock-Dominant Minerals-CharacteristicsLimestone-Marble-Calcite-Interlocking grains. Fizzes in weak acidQuartz-Sandstone-Quartzite-Quartz Sugary textureShale-Hornfels (Spotted Rock) - Micas-Dark colourRegional Metamorphism (name based on degree of metamorphism)

Texture-Rock Name-CharacteristicsSlatey-Slate-Splits easily into sheetsBetween slate and schistose-Phylitte-Silky lustre, splits into wavy sheetsSchistose-Schist-Pearly looking. Silky to touchGneissic-Gneiss-Wavy, white and dark layersCauses of MetamorphismContactCaused by heating from an external source. Contact metamorphism occurs next to an igneous body. The degree of metamorphism decreases away from the body. This occurs at fairly shallow depths, as temperature not pressure is the dominating factor.RegionalRegional metamorphism is caused by high pressure and temperatures usually duringmountain building (oregenesis). The extremes of regionally metamorphic rocks are a high pressure, low temperature rock (called a blueschist) and a high pressure and very high temperature rock (called a granulite). If the rock is heated to

the point of melting, but doesn'tnactually melt, it is called a migmatite.Introduction to Igneous PetrologyIntroductionIgneous rocks are formed form the cooling of molten rock, magma. They are crystalline, which means they are made up of crystals joined together. There are manydifferent types of igneous rocks but they fall into two (very) broad categories;intrusive and extrusive. Intrusive rocks are igneous rocks which form at depth.They cool slowly, taking tens of thousand of years to cool. They have large crystals, tens of millimetres in size. Extrusive rocks are those which have eruptedfrom volcanoes. They have very small crystals, not visible to the naked eye, asthey cooled quickly. Of course there is every grain size possible in between these two extremes.Chemistry

The chemistry of igneous rocks is quite complicated. It depends on two things; evolution and silica saturation. In this tutorial we will concern ourselves withthe effect of evolution only, the silica saturation will be assumed to be constant. Igneous rocks evolve as they cool. This process is called differentiation. The mechanism for this process is as follows:Liquid rich in minerals A, B and C.Remove mineral A as it crystallises at a higher temperature than B and C. Liquidis relatively enriched in minerals B and C.Remove mineral B as it crystallises at a higher temperature than C. Liquid is now completely mineral C. The minerals are removed in order of Bowen's Reaction Series.As you can see, if you remove olivine, the magma will become more enriched in pyroxenes etc. This process continues until only quartz is left. This leads us to

the following, simple, identification.Textures & NamesIgneous rocks have many textures which tell us about their cooling histories and/or chemistry. In general rocks which have cooled rapidly are fine grained, thatis with grains which are not visible to the naked eye. Rocks which have cooledslowly have large grains, sometimes as large as several centimetres across:Textures & NamesThis size variation arises as grains grow around a nucleus of some sort, i.e.a minute grain. The slower the cooling the more time grains have to grow and amalgamate. Grains which show their true shape are said to be euhedral. Grains which s


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how no shape are anhedral. Using this information, the order of grain growth canbe worked out. For example, a rock may have large euhedral quartz grains, whichare surrounded by anhedral feldspar. The quartz grew first as it had space, thefeldspar then grew around the quartz.Other features seen are:Porphyritic texture-large grains (phenocrysts) surrounded by much finer grains (groundamss). This implies that the large grains grew slowly at depth, the magmawith the grains in it, then rose up in the crust, cooling much more quickly forming the fine grains (the matrix).Exsolution-occurs within grains on certain minerals (pyroxenes and feldspars). This can give an indication of pressure and hence depth.Xenoliths-bits of the rock into which the magma intrudedCumulate layer-when a mineral grows which is denser than the magma, it will sinkto the base of the chamber causing a cumulate layer. Minerals may form from liquid trapped between the grains-interstitial minerals.Igneous Rock FormationsIgneous rock bodies are either intrusive or extrusive. Extrusive bodies are lavaflows. If these occur under water they form pillow lavas. On land they can formlava tubes, aa (pronounced ah-ah and looks blocky) or pahoehoe (which looks ropey).Intrusive bodies form when magma is injected into existing rock layers. A dyke is a body which cuts across the country (host) rock. A sill is parallel to the bedding layers. The baked margin is an area in the country rock, in contact with the igneous body, which has been thermally metamorphosed. The chilled margin is t

he area in an igneous body, in contact with the country rock, which cooled quicker than the rest of the rock due to the temperature difference between the magmaand the country rock. These features are not always visible. The scale of thesebodies is from millimetres to tens or even hundreds of metres.The largest of igneous bodies is a pluton or batholith. These are massive, hundreds of kilometres in size. The moors of Cornwall and Devon are outcrops of a massive batholith.

Identification of Sedimentary Rocks

Hardness Grain Size Composition Other Rock Typehard coarse clean quartz white to brown Sandstonehard coarse quartz and feldspar usually very coarse Arkosehard or soft mixed mixed sediment with rock grains and clay gray ordark and dirty Wacke/Graywackehard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomeratehard or soft mixed mixed rocks and sediment sharp pieces in finer sediment matrix Brecciahard fine very fine sand; no clay feels gritty on teeth Siltstone

hard fine chalcedony no fizzing with acid Chertsoft fine clay minerals foliated Shalesoft fine carbon black; burns with tarry smoke Coalsoft fine calcite fizzes with acid Limestonesoft coarse or fine dolomite fizzing with acid unless powderedDolomite rocksoft coarse fossil shells mostly pieces Coquinavery soft coarse halite salt taste Rock Saltvery soft coarse gypsum white or pink GypsumSedimentary rocks are the second great rock class. Whereas igneous rocks are bor


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and planktonic forms, in addition to those brought in from tributary river/stream systems, and from soil in-wash), The best preservation conditions in terms ofdiatoms are those with any mixture of fine grained, anaerobic, and slightly acidic sediments.Sampling is most frequently carried out by random core samples of a given area,as this preserves changes in the diatom assemblages over time. Where cores are sampled from beneath existing lakes, care should be taken to disturb the sediment-water interface as little as possible. Often a rich organic mud called gyttja (typical of interglacial periods), will have accumulated, consisting of mainly faecal debris, animal and plant remains, along with some clastic component (sand/silt/clay), Will retain a record of the most recent diatom activity.Uses Of DiatomsIn general diatoms can be used to trace a variety of environmental phenomena, from changes in sea level (whether brought about by climate change or tectonic activity), breaches of coastal barriers (as a result of storms and/or sea-level rise), to the evaporation of lakes (increasing salinity determining diatom assemblages), Below is an outline of their most prevalent uses.Indicators of SalinityMarine: Some species are restricted to a very narrow range of salinities and areknow as stenohaline species, others have no such restrictions and are known ascosmopolitan species. As a result, this causes zonation, which is particularly evident in estuaries, where a spectrum (and a gradient for such a spectrum) can be calculated from coastal to offshore species. This has applications in determining palaeo-fluvial environments, and sediment focusing.

Freshwater: Some freshwater species will tolerate a little salt, and are known as halophilic, occurring in coastal lakes, or where the groundwater is rich in salts. However most freshwater species are stenohaline and will not tolerate salt.Indicators of Productivity (Trophic Status)There are several ways of deducing palaeotrophic status using diatoms:Total Diatom Count-This is relatively simple, the more diatoms there are in yoursample, the more productive a given body of water is:Centric: Pennate Ratio-The more centrics there are in your sample, the more productive the environment is (With the exception of a species called Cyclotella.).Indicator Species-Certain species are typical of certain conditions, for exampleStephanodiscus is typical of eutrophic (abundant nutrient) conditions, and Tabellaria of oligotrophic (very low nutrient) conditions.

Planktonic: Non-planktonic Ratio-Planktonic forms are more common in eutrophic lakes.Diversity Indicators-A low overall diversity amongst diatoms indicates stressfulconditions, for example extreme trophic status (hyper-oligotrophic or hyper-eutrophic). However this could also indicate a source of pollution etc.Indicators of Palaeo-pHThis perhaps the most important and most widely used application of diatom studies.Diatoms are highly sensitive to pH and can illustrate differences of as little as 0.1 pH units. To accomplish this species are classified as either:Acidobiontic (Acid Living) pH < 7Acidophilous (Acid Preferring) pH 7Circumneutral pH = 7

Alkaliphilous (Alkali Preferring) pH 7Alkalibiontic (Alkali Living) pH > 7This method is highly dependent upon knowing the pH preference for all of the diatoms present, as the percentage of each of the above groups is measured and theratios used to calculate a log index of the given population. With the use of some complicated mathematics this, in turn, can then be used to determine the palaeo-pH. Obviously, it is not always possible to know the preference of all of the species in your sample, and therefore this method can not always be applied.Indicators of Palaeo-pHThis perhaps the most important and most widely used application of diatom studi


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es.Diatoms are highly sensitive to pH and can illustrate differences of as little as 0.1 pH units. To accomplish this species are classified as either:Acidobiontic (Acid Living) pH < 7Acidophilous (Acid Preferring) pH 7Circumneutral pH = 7Alkaliphilous (Alkali Preferring) pH 7Alkalibiontic (Alkali Living) pH > 7This method is highly dependant upon knowing the pH preference for all of the diatoms present, as the percentage of each of the above groups is measured and theratios used to calculate a log index of the given population. With the use of some complicated mathematics this, in turn, can then be used to determine the palaeo-pH. Obviously, it is not always possible to know the preference of all of the species in your sample, and therefore this method can not always be applied.Indicators of Palaeo-temperatureDiatoms are not very useful in determining changes in palaeo-temperature, due tothe fact that the large majority of species will tolerate very wide ranges of temperature, typically from 0 C to 20 C.That said, different assemblages are present when comparing warm and cold waters. However, this is almost certainly due to other overriding factors such as: Incident solar radiation, water chemistry, pH, and nutrient availability.Difficulties in Interpreting Diatom SamplesNot all diatoms present in a body of water may settle out, they can be lost viaoutflows, dissolve, be crushed or eaten. In the best case scenario your assembla

ge is simply incomplete, or comparatively low in overall abundance. In the worstcase scenario the ratios of different diatoms may be completely skewed (for example planktonic forms with their oil-filled globules may be more prone to out-washing),Samples may contain diatoms washed in from outside your sample area, from soils,animal droppings, or tributaries. The sample becomes augmented, and in the worst case scenario may include indicator species contrary to the actual palaeo-environmental conditions.There may be insufficient silica dissolved in the body of water for diatoms to produce robust, preservable frustules, resulting in a complete absence in your sample.Taxonomy, especially in poorly preserved specimens, can often be difficult resulting in mis-identification and a chain of consequent errors.

The ecology is not well known for all species, causing problems and/or errors with interpretation.

Ooid FormationIntroductionOoids are spherical or ellipsoid concretions of calcium carbonate, usually lessthan 2mm in diameter (Donahue, 1969; Tucker and Wright, 1990). There have been examples in the Neoprotozoic of ooids that are 16mm in diameter (Sumner, 1993), but all modern ooids are 2mm or less.The interior of an ooid is usually composed of a nucleus, which is surrounded by

a cortex of calcite or aragonite crystals that are arranged radially, tangentially or randomly. These crystals are arranged in concentric lamina. The nucleus can be a shell fragment, quartz grain or any other small fragment (including an aragonite/calcite amalgamation).The formation of these objects has been speculated from the early 19th Century and ideas for their origin range from crinoid eggs, insect eggs to the present day explanation of precipitated layers of CaCO3 (Simone, 1981).Recent ooids are forming today in places such as the Bahamas (Tucker and Wright,1990; Newell et al. 1966) and Shark Bay, Australia and are all composed of aragonite.


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Life CycleOoids do not form continuously; instead they go through stages of growth and rest (Davis et al. 1978). Davies et al (1978). Describe the typical life cycle of aBahamian ooid:Suspension Growth Phase Nuclei introduced into a suitable location, with enoughturbulence to keep them in suspension and water that is supersaturated in CaCO3,will induce a short lived inorganic precipitation of calcium carbonate on theirsurfaces. The precipitation is stopped by crystal poisoning, which is the addition of Mg2 + or H + onto the surface. If the proto-ooids remain in this environment the outer coating will be lost due to attrition. This means the suspension phase is short lived, but may be repeated several times.Temporary Resting Phase Coated nuclei resting in the marine environment will quickly equilibrate with the surrounding fluid. Removal of the poisonous ions will reactivate the coated surface in such conditions. However, not all poisonous ions are removed, so after several growth and temporary resting stages have been completed a third stage is required.Sleeping Stage A new surface is required in order to form a new coating. This membrane is probably organic in origin. Experiments show that this takes 1 ? 3 weeks to form. The membrane forms a new, stable substratum for new CaCO3 precipitation.The timing of these stages means that an ooid spends only 5% of its time actually growing; the rest is spent sleeping (Davis et al. 1978; Bathurst, 1967).FormationAs can be seen from the life cycle, the following factors will have an affect oo

id growth: (Monoghan and Lytle, 1956; Newell, 1960; Bathurst, 1967; Davis et al.1978; Deelman, 1978; Heller, 1980; Simone, 1981; Sumner and Grotzinger, 1993):Supersaturation: The supersaturation of the seawater is of vital importance (Monoghan and Lytle, 1956). Monoghan and Lytle (1956) investigated the effect of CO3concentration on the formation of ooids. They found that the concentration needed to be above 0.002 moles/litre and below 0.0167 moles/litre for ooids to formsuccessfully. Below 0.002 moles/litre only aragonite needles or poor ooids formed. Above 0.0167 moles/litre the ooids formed an amorphous mass. Other authors have stressed the importance of supersaturation, but they give no quantitative information (Bathurst, 1967; Davies et al. 1978; Simone, 1981).Nuclei: The type of nuclei affects the rate of growth and the size of each lamination (Davies et al. 1978). Organic coating on the nuclei give faster and longerprecipitation, while using oxidised quartz show much slower and shorter precipi

tation. Davies et al (1978). Show their results as a change of pH (a negative pHchange is assumed to indicate precipitation), rather than growth or precipitation rates.Agitation: The agitation an ooid undergoes must be enough to keep it in suspension for the growing phase followed by removal to a non-supersaturated fluid (therest phase) (Newell, 1960; Davies et al. 1978; Heller, 1980). Davies et al (1978). Conducted a study using two different speeds of water current to test this: 5Cm/s and 10 cm/s. The ooids were kept in suspension by this water flow, and in other experiments involving horizontal shaking and tumbling motion formed, the ooids were non-existent or more like those formed in non-agitated water in the presence of organic compounds. In all cases of different nuclei the larger water current increase precipitation rates, but the time that precipitation changed depending on the nuclei type. Agitation may also control ooid size (Sumner and Grotz

inger, 1993). As the ooid grows the mass lost per impact with another object increases as the cube of the radius. The mass gained from growth is proportional tothe square of the radius. Eventually, the mass loss will equal or exceed the mass gained, limiting the size of the ooid. Sumner and Grotzinger (1993) performednumerical modelling on ooid formation. Their model gave a higher ooid radius inhigher velocity flows, with a decrease that looks like an exponential or a power law with decreasing velocity (Sumner and Grotzinger, 1993, their fig 6). Theydid not include the impact of ooids to limit size. The agitation can come from waves or tidal movements. Storms provide that mixing of ooids in the rest stage and those that can no longer precipitate. There is some change in crystal orienta


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tion with the amount of agitation (Donahue, 1969). Ooids can form in quiet waters, but organic CaCO3 precipitation is needed for them to form (Suess and Fatterer, 1972). These ooids will show radial crystals. Ooids formed in agitated watershave crystals arranged tangentially. The change between suspension to bedload transport may also initiate this change (Deelman, 1978).Location: The location off ooid formation is important. They must be kept in thesame area throughout the formation, in order that their life cycle can be completed (Simone, 1981).Water Depth: Most ooids form in water less than 2m deep (Simone, 1981), but thismay have more to do with wave agitation and tidal movements than water depth itself. Newell et al (1960). Surveyed sediment at various depths and calculated the % fraction of ooids in the sediment. All the sediments that are near 100% ooids are formed with 8m of the surface.

MineralsGeologists know about thousands of minerals locked in rocks, but when rocks areexposed at the surface and weather away, less than 10 minerals remain. They arethe ingredients of sediment, which in turn becomes sedimentary rock. When the mountains crumble to the sea, all of their rocks, whether igneous, sedimentary or

metamorphic, break down. Physical or mechanical weathering reduces the rocks tosmall particles. These break down further by chemical weathering in water and oxygen. A very small number of minerals can resist indefinitely: Zircon is one andnative gold is another. Quartz resists for a very long time, which is why sand,being nearly pure quartz, is so persistent, but given enough time even quartz dissolves into silicic acid, H4SiO4.But most of the silicate minerals produce solid residues after chemical weathering. Silicate residues are what make up the minerals of the Earth's land surface.The olivine, pyroxenes and amphiboles of igneous or metamorphic rocks react withwater and leave behind rusty iron hydroxides. These are an important ingredientin soils but uncommon as solid minerals. They also add brown and red colors tosedimentary rocks.

Feldspar, the most common silicate mineral group and the main home of aluminum in minerals, reacts with water too. Water pulls out silicon and other major cations (positive ions) except for aluminum. The feldspar minerals thus turn into hydrated aluminosilicates that is, clays.Amazing ClaysClay minerals are not much to look at, but life on Earth depends on them. At themicroscopic level, clays are tiny flakes, like mica but infinitely smaller. Atthe molecular level, clay is a sandwich made of sheets of silica (SiO4) tetrahedra and sheets of magnesium or aluminum hydroxide (Mg (OH) 2 and Al (OH) 3). Someclays are a proper three-layer sandwich, a Mg/Al layer between two silica layers, while others are open-face sandwiches of two layers.What makes clays so valuable for life is that with their tiny particle size andopen-faced construction, they have very large surface areas and can readily acce

pt many substitute cations for their Si, Al and Mg atoms. Oxygen and hydrogen are available in abundance. From the viewpoint of microbes, clay minerals are likemachine shops full of tools and power hookups. Indeed, even the building blocksof life amino acids and other organic molecules are enlivened by the energetic,catalytic environment of clays.The Makings of Clastic RocksBut back to sediments. With quartz and clay, the overwhelming majority of surface minerals, we have the ingredients of mud. Mud is what geologists call a sediment that is a mixture of particle sizes ranging from sand (visible) to clay (invisible), and the world's rivers steadily deliver mud to the sea and to large lake


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s and inland basins. That is where the clastic sedimentary rocks are born, sandstone and mudstone and shale in all their variety.The Chemical PrecipitatesWhen the mountains were crumbling, much of their mineral content dissolved. Thismaterial reenters the rock cycle in other ways than clay, precipitating out ofsolution to form other surface minerals.Calcium is an important cation in igneous rock minerals, but it plays little part in the clay cycle. Instead calcium remains in water, where it affiliates withcarbonate ion (CO3). When it becomes concentrated enough in seawater, calcium carbonate comes out of solution as calcite. Living organisms can extract it to build their calcite shells, which also become sediment.Where sulfur is abundant, calcium combines with it as the mineral gypsum. In other settings, sulfur captures dissolved iron and precipitates as pyrite.There is also sodium left over from the breakdown of the silicate minerals. Thatlingers in the sea until circumstances dry up the brine to a high concentration, when sodium joins chloride to yield solid salt, or halite. And what of the dissolved silicic acid? That precipitates underground, from deeply buried fluids, as the silica mineral chalcedony. Thus every part of the mountains finds a new place in the Earth.Minerals, Gemstones & Mineral ResourcesWhat Is a Mineral?A mineral is any substance with all of four specific qualities.Minerals Are Natural: Substances that form without any human help.Minerals Are Solid: Substances that don'tndroop or melt or evaporate.

Minerals Are Inorganic: Substances that aren'tncarbon compounds like those foundin living things.Minerals Are Crystalline: Substances that have a distinct recipe and arrangementof atoms.Unnatural MineralsUntil the 1990S, mineralogists could propose names for chemical compounds that formed during the breakdown of artificial substances, things found in places likeindustrial sludge pits and rusting cars (although iron rust is the same as thenatural minerals hematite, magnetite and goethite). That loophole is now closed,but there are minerals on the books that aren'tntruly natural.Soft MineralsTraditionally, native mercury is considered a mineral, even though the metal isliquid at room temperature. At about 40 degrees below zero, mercury solidifies a

nd forms crystals like other metals. So there are parts of Antarctica where mercury is unimpeachably a mineral.For a less extreme example, consider the mineral ikaite, a hydrated calcium carbonate that forms only in cold water. It degrades into calcite and water above 8degrees Celsius. It is significant in the polar regions, the ocean floor and other cold places, but you can'tnbring it into the lab except in a freezer.Ice is a mineral, even though it isn'tnlisted in the mineral field guide. But when ice collects in large enough bodies, it flows in its solid state, that's whatglaciers are. And salt (halite) behaves similarly, rising underground in broaddomes and sometimes spilling out in salt glaciers. Indeed, all minerals, and therocks they are part of, slowly deform given enough heat and pressure. That's what makes plate tectonics possible. So in a sense, no mineral is really solid except maybe diamond.

Other minerals that aren'tnquite solid are instead flexible. The mica minerals are the best-known example, but molybdenite is another. Its metallic flakes can be crumpled like aluminum foil. And of course the asbestos mineral chrysotile isstringy enough to weave into cloth.Organic MineralsThe rule that minerals must be inorganic may be the strictest one. The substances that make up coal, for instance, are different kinds of hydrocarbon compoundsderived from cell walls, wood, pollen and so on. These are called macerals instead of minerals (for more see Coal in a Nutshell). But if coal is squeezed hard enough for long enough, the carbon sheds all its other elements and becomes graph


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ite. Even though it is of organic origin, graphite is a true mineral, carbon atoms arranged in sheets. Diamond, similarly, is carbon atoms arranged in a rigid framework. After some 4 billion years of life on Earth, it's safe to say that allthe world's diamonds and graphite are of organic origin even if they aren'tnstrictly speaking organic.Amorphous MineralsA few things fall short in crystallinity, hard as we try. Many minerals form crystals that are too small to see under the microscope. But even these can be shown to be crystalline at the nano-scale using the technique of X-ray powder diffraction, though, because X-rays are a super-short-wave type of light that can image extremely small things.Having a crystal form means that the substance has a definite recipe, or chemical formula. It might be as simple as halite's (NaCl) or complex like, say, epidote (Ca2Al2 (Fe3 +, Al) (SiO4) (Si2O7) O (OH) ), but if you were shrunk to an atom's size, you could tell what mineral you were seeing by its molecular makeup andarrangement.But a few substances fail the X-ray test. They are truly glasses or colloids, with a fully random structure at the atomic scale. They are amorphous, scientificLatin for formless. These get the honorary name mineraloid.Mineraloids are a small club: Strictly speaking it includes only opal and lechatelierite. Opal is a nearly random combination of silica (SiO2, the same as quartz) and water formed under near-surface conditions, while lechatelierite is a quartz glass formed by the shock of a meteorite impact or lightning striking the ground.

Other substances considered mineraloids include the gemstones jet and amber, which are respectively high-quality fossils of coal and tree resin. Pearl goes heretoo, although I disagree because by that logic, seashells should be included. The last mineraloid is rather like the rusty car I mentioned earlier: Limonite isa mixture of iron oxides that, while it may assume the shape of a proper iron-oxide mineral, has no structure or order whatever.Examine MineralSteps to Mineral Identification: The first thing to do is to observe and test your mineral. Use the largest piece you can find, and if you have several pieces,make sure sure that they are all the same mineral. Examine your mineral for allof the following properties, writing down the answers. After that you'llube ready to take your information to the right place.Step 1: Pick Your Mineral

Step 2: Luster. Luster is the way a mineral reflects light and the first key step in mineral identification. Look for luster on a fresh surface. The three majortypes of luster are metallic, glassy (vitreous) and dull. A luster between metallic and glassy is called adamantine, and a luster between glassy and dull is called resinous or waxy.Step 3: Hardness. Use the 10 - point Mohs hardness scale. The important hardnesses are between 2 and 7. For this you'lluneed your fingernail (hardness about 2),a coin (hardness 3), a knife or nail (hardness 5.5) and a few key minerals.Step 4: Color. Color is important in mineral identification, but it can be a complicated subject. Experts use color all the time because they have learned the usual colors and the usual exceptions for common minerals. If you'reua beginner,pay close attention to color but do not rely on it. First of all, be sure you aren'tnlooking at a weathered or tarnished surface, and examine your specimen in g

ood light. Color is a fairly reliable indicator in the opaque and metallic minerals for instance the blue of the opaque mineral lazurite or the brass-yellow ofthe metallic mineral pyrite. In the translucent or transparent minerals, color is usually the result of a chemical impurity and should not be the only thing youuse. For instance, pure quartz is clear or white, but quartz can have many other colors.Step 5: Other Mineral Properties. Taste is definitive for halite (rock salt), ofcourse, but a few other evaporite minerals also have distinctive tastes. Just touch your tongue to a fresh face of the mineral and be ready to spit after all it's called taste, not flavor. Don'tnworry about taste if you don'tnlive in an ar


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ea with these minerals. Fizz means the effervescent reaction of certain carbonate minerals to the acid test. For this test, vinegar will do. Heft is how heavy amineral feels in the hand, an informal sense of density. Most rocks are about three times as dense as water, that is, they have a specific gravity of about 3.Make note of a mineral that is noticeably light or heavy for its size.Step 6: Look It Up. Now you are ready for mineral identification. Once you haveobserved and noted these mineral properties, you can take your information to abook or to an online resource. Start with my table of the rock-forming minerals,because these are the most common and the ones you should learn first. Each mineral's name is linked to a good photograph and notes to help you confirm the identification. If your mineral has metallic luster, go to my Minerals with Metallic Luster gallery to see the most likely minerals in this group. If your mineralis not one of these, try the sources in the Mineral Identification Guides category.

Minerals to Gemstones

Mineral GemstoneAlbite MoonstoneOlivine Chrysolite, PeridotAlmandine GarnetOpal OpalAlmandine-Pyrope Garnet Rhodolite

Orthoclase Feldspar MoonstoneAmber AmberPlagioclase Feldspar MoonstoneAndalusite AndalusitePyrope GarnetAndradite Demantoid GarnetQuartz Amethyst, Ametrine, Cairngorm, Citrine, MorionApatite ApatiteRhodochrosite RhodochrositeBenitoite BenitoiteScapolite ScapoliteBeryl Aquamarine, Beryl, Emerald, Goshenite, Heliodore, MorganiteSinhalite Sinhalite

Brazilianite BrazilianiteSodalite SodaliteChalcedony Agate, Aventurine, Bloodstone, Carnelian, Chrysoprase, Heliotrope, Jasper, Onyx, SardSpessartine Mandarin GarnetChrysoberyl Alexandrite, ChrysoberylSphene TitaniteCordierite Cordierite, Dichroite, IoliteSpinel Pleonast, RubicelleCorundum Ruby, SapphireSpodumene Hiddenite, KunziteDiamond DiamondSugilite Sugilite

Diopside Chrome Diopside, ViolanTaaffeite TaaffeiteGrossularite Hessonite, Tsavorite GarnetTopaz TopazJadeite JadeTourmaline Achroite, Dravite, Indigolite/Indicolite, Rubellite, Schorl, VerdeliteLazurite Lapis LazuliTurquoise TurquoiseMalachite Malachite


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Uvarovite Garnet, UvaroviteMicrocline Feldspar Amazonite, MoonstoneZircon ZirconNephrite JadeZoisite TanzaniteOligoclase Feldspar Sunstone

MetamorphismThe Four Agents of Regional MetamorphismHeat and pressure usually work together, because both rise as you go deeper in the Earth. The clay minerals of sedimentary rocks, in particular, respond to hightemperatures and pressures. Clays are surface minerals, which form as feldsparand mica break down in the conditions at the Earth's surface.With heat and pressure they slowly return to mica and feldspar. Thus the sedimentary rock shale metamorphoses first into slate, then into phyllite, then schist.The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented as the sedimentary rock sandstone turns to quartzite. Intermediate rocks that mix sand and clay. Mudstones. Metamorphose into gneiss. The sedimentary rock limestone recrystallizes and becomes marble.Fluids are the most important agent of metamorphism. Every rock contains some wa

ter, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated byclay minerals as they change back to feldspar and mica. This water can become socharged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica (forming chalcedony) or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. That process, the interaction of rock with chemically active fluids, is called metasomatism.Strain refers to any change in the shape of rocks due to the force of stress. Asfluids form and move in buried rocks, new minerals grow with their grains oriented according to the direction of pressure. Where the strain makes the rock stretch (shear strain), these minerals form layers. In most metamorphic rocks the la

yers are made of mica. The presence of mineral layers is called foliation and isimportant to observe when identifying a metamorphic rock. As strain increases,the foliation becomes more intense, and the mineral sort themselves into thickerlayers. That's what gives schist and gneiss their foliation.Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy, and the result iscalled migmatite. With further metamorphism, rocks can be turned into somethinghard to tell from plutonic granites. These kinds of rocks give joy to experts because of what they say about deep-seated conditions during things like plate collisions. The rest of us can only admire the laboratory skills needed to make sense of such rocks.What I'veIdescribed is how regional metamorphism affects sedimentary rocks. Igneous rocks give rise to a different set of minerals and metamorphic rock types; t

hese include serpentinite, blueschist, greenschist and other rarer species suchas eclogite. If you'reua mineral collector it's worth your while to learn aboutthese, but they aren'tnfound in most parts of the world.Contact or Local MetamorphismA lesser type of metamorphism, important in specific localities, is contact metamorphism. This usually occurs near igneous intrusions, where hot magma forces itself into sedimentary strata. The rocks next to the lava invasion are baked intohornfels, another subject for specialists. Lava can rip chunks of country rockoff the channel wall and turn them into exotic minerals, too.Underground coal fires can also cause mild contact metamorphism of the same degr


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ee as occurs when baking bricks.

Identification of Metamorphic Rocks

Foliation Grain Size Hardness Usual Color Other Rock Typefoliated fine soft dark tink when struck Slatefoliated fine soft dark shiny; crinkly foliation Phyllitefoliated coarse hard mixed dark and light wrinkled foliation; often has large crystals Schistfoliated coarse hard mixed banded Gneissfoliated coarse hard mixed distorted melted layers Migmatite

foliated coarse hard dark mostly hornblende Amphibolite

nonfoliated fine soft greenish shiny, mottled surface Serpentinitenonfoliated fine or coarse hard dark dull and opaque colors, found near intrusions Hornfelsnonfoliated coarse hard red and green dense; garnet and pyroxeneEclogitenonfoliated coarse soft light calcite or dolomite by the test

Marblenonfoliated coarse hard light quartz (no fizzing with acid) QuartziteMetamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditionsunderground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur inmetamorphic ( shape-changed ) rocks. Metamorphism acts at two scales, the regional scale and the local scale.

Identification of Igneous Rocks

Grain Size Usual ColorOther CompositionRock Type finedark glassy appearancelava glass Obsidianfine lightmany small bubbles lava froth from sticky lavaPumice finedark many large bubbleslava froth from fluid lava Scoriafine or mixed light

contains quartz high-silica lavaFelsite fine or mixedmedium between felsite and basaltmedium-silica lava Andesitefine or mixed darkhas no quartz low-silica lavaBasalt mixedany color large grains in fine-grained matrixlarge grains of feldspar, quartz, pyroxene or olivine Porphyrycoarse light


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wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxeneGranite coarselight like granite but without quartzfeldspar with minor mica, amphibole or pyroxene Syenitecoarse medium to darklittle or no quartz low-calcium plagioclase and dark mineralsDiorite coarsemedium to dark no quartz; may have olivinehigh-calcium plagioclase and dark minerals Gabbrocoarse darkdense; always has olivine olivine with amphibole and/or pyroxenePeridotite coarsedark densemostly pyroxene with olivine and amphibole Pyroxenitecoarse greendense at least 90% olivineDunite very coarseany color usually in small intrusive bodiestypically granitic PegmatiteChemical Sedimentary RocksThese same ancient shallow seas sometimes allowed large areas to become isolatedand begin drying up. In that setting, as the seawater grows more concentrated,minerals begin to come out of solution (precipitate), starting with calcite, the

n gypsum, then halite. The resulting rocks are certain limestones or dolomites,gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan. In some cases chert can also form byprecipitation. This usually happens below the sediment surface, where differentfluids can circulate and interact chemically.Diagenesis: Underground ChangesAll kinds of sedimentary rocks are subject to further changes during their stayunderground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals.These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphosis (although there is no well-defined boundary between the two).The most important types of diagenesis involve the formation of dolomite mineral

ization in limestones, the formation of petroleum and of higher grades of coal and the formation of many types of ore bodies. The industrially important zeoliteminerals also form by diagenetic processes.Sedimentary Rocks Are StoriesThe beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils, marks left by water currents, mudcracks or more subtle features seen under the microscope or in the lab.From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: Clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desertsand, organic rocks in peat bogs or lake beds, and evaporites in playas. Theseare called continental or terrigenous (land-formed) sedimentary rocks.Sedimentary rocks are rich in geologic history of a special kind. While igneous

and metamorphic rocks also have stories, they involve the deep Earth and requireintensive work to decipher. But in sedimentary rocks you can recognize, in verydirect ways, what the world was like in the geologic past.

Gemstones to Minerals


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Gemstone MineralAchroite TourmalineKunzite SpodumeneAgate ChalcedonyLapis Lazuli LazuriteAlexandrite ChrysoberylMalachite MalachiteAmazonite Microcline FeldsparMandarin Garnet SpessartineAmber AmberMoonstone Orthoclase, Plagioclase, Albite, Microcline FeldsparsAmethyst QuartzMorganite BerylAmetrine QuartzMorion QuartzAndalusite AndalusiteOnyx ChalcedonyApatite ApatiteOpal OpalAquamarine BerylPeridot OlivineAventurine ChalcedonyPleonast Spinel

Benitoite BenitoiteQuartz QuartzBeryl BerylRhodochrosite RhodochrositeBloodstone ChalcedonyRhodolite Almandine-Pyrope GarnetBrazilianite BrazilianiteRubellite TourmalineCairngorm QuartzRubicelle SpinelCarnelian ChalcedonyRuby CorundumChrome Diopside Diopside

Sapphire CorundumChrysoberyl ChrysoberylSard ChalcedonyChrysolite OlivineScapolite ScapoliteChrysoprase ChalcedonySchorl TourmalineCitrine QuartzSinhalite SinhaliteCordierite CordieriteSodalite SodaliteDemantoid Garnet AndraditeSpinel Spinel

Diamond DiamondSugilite SugiliteDichroite CordieriteSunstone Oligoclase FeldsparDravite TourmalineTaaffeite TaaffeiteEmerald BerylTanzanite ZoisiteGarnet Pyrope, Almandine, Andradite, Spessartine, Grossularite, Uvarovite


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Titanite SpheneGoshenite BerylTopaz TopazHeliodore BerylTourmaline TourmalineHeliotrope ChalcedonyTsavorite Garnet GrossulariteHessonite GrossulariteTurquoise TurquoiseHiddenite SpodumeneUvarovite UvaroviteIndigolite/Indicolite TourmalineVerdelite TourmalineIolite CordieriteViolan DiopsideJade Nephrite or JadeiteZircon ZirconJasper Chalcedony

Gemstones and Precious StonesGemstones are the sexy minerals. If minerals are like different sorts of people,gemstones are the supermodels. If mineralogists are like zookeepers, who collect and classify all the different animals, gemologists are like butchers, who focus on the edible ones. Where the mineralogist asks What variety of cow is this? the gemologist asks Where's the beef?Gemstone Fanciers versus Mineral CollectorsJust as beeves and cows are different names for the same thing, many gemstones have names that differ from their proper mineral names. Olivine is an important rock-forming mineral, for example, but as a gemstone it's called peridot. To keepthe two sets of names straight, use the Gemstones to Minerals tables.There are two ways of appreciating the mineral kingdom.

The collector of minerals loves their natural crystal form, chemical variety, fluorescence, rarity the personality of minerals in themselves. If you'reua mineral-collecting kind of person, you might find a place like Emeralds appealing, which sells only uncut emerald crystals.The fancier of gemstones is in love with what makes minerals sexy: Their purity,color, size, optical effects and value in a word, their beauty. The rest of this article is for fanciers.Of course these categories overlap. That's why I have a big Gemstones category that gives you a peek over the fence from the mineral collector's side.Digging GemstonesI suppose there could be a third reaction to browsing all these jewels Where canI dig up my own? There are gemstone mines all over the place. One place they'reyconcentrated in is the Franklin district of North Carolina. One of Carly Wickell'

s favorites is the Sheffield Ruby Mine. But most mines are generally enriched the old-fashioned term is salted with extra stones. If you don'tnmind that, or ifyou'reutaking children with you, then these places are fine. To do better, jointhe rockhounds near you and follow them around:The ultimate gemstone fanatic dreams of opening a mine. People have found valuable things in their own yards, after all. You might not have to move to Franklin.For a real-life example, read about Scott Klein's fresnoite mine deep in the California Coast Range.MineralogyThere are three main minerals that form carbonates:


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Calcite (CaCO3), which comes in high magnesium and low magnesium forms.Aragonite (CaCO3), which has a different structure to calcite.Dolomite (CaMg (CO3) 2), a magnesium rich carbonate produced by diagenesis.Only low magnesium calcite is stable at surface pressure and temperatures. It istherefore the most common mineral in ancient carbonates. However, most modern carbonates are composed of aragonite as this is the mineral that most biologicalorganisms create to make their shells or skeletons. Examples of organisms that produce aragonite shells are bivalves (sea shells), gastropods (snails) and Halimeda (a green algae). Organisms that produce a calcite shell include brachipods (a rare type of sea shell) and ostrocods (a small crustacean).ComponentsCarbonates can be made of several components. These are:Bioclasts: Bioclasts are fragments of dead sea creatures. These include shells and corals. The creatures precipitate the carbonate in order to produce some kindof structure.Ooids: Ooids are rounded grains formed by precipitation of calcite around a nucleus to produce concentric circles (Figure 3). They form in warm, shallow waters,with a strong tidal currents. Wave action may also contribute to their near-spherical shape.Peloids: Peloids are sand sized grains (100 ? 150 micrometers) of micro-crystalline carbonate. They are generally rounded or sub-rounded. They originate from fecal pellets, algae and mud clasts. They are sometimes found clumped together, ina formation known as a grapestone.Intraclasts: Intraclasts are clast of other limestone that appear in younger lim

estones. They can be quite difficult to distinguish at times, as they may be made of a similar rock as that which encases it. For example, hardgrounds can fromwhen sea water flows through carbonate sediment, lithifying it rapidly. Subsequently, the hardground may be broken up and incorporated into the surrounding sediment.Micrite: Micrite is microcystalline carbonate mud, with grains less than 4 micrometers.Sparite: Sparite is coarser than micrite, with a grain size of more than 4 micrometers and is crystalline. Both micrite and sparite form the matrix or cement incarbonate rocks.

The Origin and Early Evolution of BirdsIntroductionBirds are phylogenetically considered to be members of the theropod dinosaurs; their closest non-avian relatives are the dromaeosaurid theropods. The first known fossil bird is Archaeopteryx, from the late Jurassic of Bavaria, Germany, which is represented by seven skeletons and a feather. There is no fossil evidence from before this time that has been proven to be of avian origin. The fossil record of modern birds began in the early Tertiary (Padian et al. 1998).There are a lot of anatomical terms used to describe the evolution of birds, therefore diagrams showing the skeletons of a theropod dinosaur, Archaeopteryx anda modern bird are shown in figure one, in order to define most of the terms used

.The Thecodont HypothesisThe thecodont hypothesis for the origin of birds is characterised by being a default option. This is because it is not due to positive correlation of charactersand taxa but due to the negative association with other taxa. It was originallythought that theropods shared more features with birds than any other group. However, at the time there was no evidence for clavicles in theropods, which are the equivalent of the furcula or wishbone in birds. It was thought that claviclescould not have been lost and then re-evolved into furcula and so a more ancientancestor for birds was sought for:


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The candidate suggested was the thecodonts from which all other archosaurs are thought to have evolved. A problem is that the archosaurs are a wastebasket group containing all archosaurian reptiles that do not fit into dinosaurs, crocodiles or pterosaurs. Hence, they have no diagnostic characters of their own and are nota good phylogenetic group. This makes it difficult to compare them with other taxonomic groups (Padian et al. 1998).The thecodont hypothesis for the origin of birds is characterised by being a default option. This is because it is not due to positive correlation of charactersand taxa but due to the negative association with other taxa. It was originallythought that theropods shared more features with birds than any other group. However, at the time there was no evidence for clavicles in theropods, which are the equivalent of the furcula or wishbone in birds. It was thought that claviclescould not have been lost and then re-evolved into furcula and so a more ancientancestor for birds was sought for. The candidate suggested was the thecodonts from which all other archosaurs are thought to have evolved. A problem is that the archosaurs are a wastebasket group containing all archosaurian reptiles that donot fit into dinosaurs, crocodiles or pterosaurs. Hence, they have no diagnosticcharacters of their own and are not a good phylogenetic group. This makes it difficult to compare them with other taxonomic groups (Padian et al. 1998).The Crocodylomorph HypothesisCrocodylomorphs include crocodiles and some Triassic-Jurassic forms that are closely related but not true crocodiles. This hypothesis has fewer problems than the thecodont hypothesis, as crocodiles are a monophyletic group, which can be compared to other taxa. The synapomorphies of birds and crocodiles have been tested

, but it has been found that most are general to the archosaurs. Even if these are accepted, there are only 15 ? 20 synapomorphies compared to over 70 with thetheropods (Padian et al. 1998).The Theropod HypothesisCladistic analysis supports this hypothesis and shows that the most closely related group (sister group) of theropods to the birds includes the dromaesaurids such as Deinonychus. There are numerous synapomorphies of the skeleton and skull that link birds and theropods. There have been found a series of skeletal changesin theropods that are considered to be avian. For example, basal theropod dinosaurs have lightly built bones and a foot reduced to three main toes, with the first held off the ground and the fifth lost. Moving through the theropod sequencetowards birds, there is a reduction and loss of manual digits four and five, increasing lightness of the skeleton, and a reduction in the number, and partial i

nterlocking, of the tail vertebrae. In coelurosaurs, which include birds, dromaeosaurids and other groups, the arms become longer and the first toe begins to rotate backwards behind the metatarsals. Fused clavicles (furcula) are apparentlybasal to Tetanurae (carnosaurs and coelurosaurs) and sternal plates are known ina variety of tetanurans. In the pelvis, the pubis and ischium begin to show a greater disparity in length. Finally, in the dromaeosaurids and Archaeopteryx, the pubis begins to point backwards instead of forwards, the anterior projection on the foot of the pubis is lost, the tail becomes even shorter and the hyperflexing wrist joint is present, which allows the action that is crucial to the flight stroke in birds. These features were passed to birds from their dinosaurian ancestors and not specifically evolved for an avian lifestyle (Padian et al. 1998).The Evolution of Feathers

The first known feathers are from a small coelurosaurian dinosaur called Sinosauropteryx, which has a row of small fringed structures along its vertebral column. Therefore, feathers are not a synapomorphy of birds, they are shared by a broader group within the theropod dinosaurs. This shows that feathers did not evolvespecifically for flight (Padian et al. 1998).Hypotheses for the original function of feathers include insulation, display andcamouflage. These are hypotheses are difficult to test, and it seems likely that feathers were used for several different purposes as they are now (Padian et al. 1998).The Cursorial Theory


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The cursorial theory is based on the evidence that Archaeopteryx was a strong, agile, biped, and from evidence that birds evolved from small, active, running predators. It has been suggested that feathers were used as nets to capture insects, but it has been shown that this action would cause the proto-bird to lose itsbalance by throwing off its angular momentum. As an alternative, it was suggested that insects were caught using the teeth with the arms held out laterally. Anincreased airfoil surface would increase lift and stability. This suggests thatflight could have begun by running, leaping and sustaining short, leg-powered glides after prey or away from predators (Padian et al. 1998).Criticisms of this include the problems of drag and needing to work against gravity. It is biomechanically easier to evolve flight from gliding than from the ground, although this does not indicate that it evolved this way. The other problem is the ground speed that would be required to reach a typical flight speed of6 ? 7 mS ? 1. Modern lizards have been reported to reach theses speeds, but it is not known whether proto-birds could reach such speeds. There has been some doubt as to whether selection could improve both the forelimb and hindlimb at the same. However, decoupling of the fore-and hindlimbs was achieved in the coelurosaurs and the tail and hindlimb were progressively decoupled in the earliest birds(Padian et al. 1998).Any model also needs to account for the evolution of the flight stroke. It has been shown that the immediate sister groups of birds, Deinonychus and the other dromaeosaurs, already had the sideways-flexing wrist joint that in birds is essential to the production of thrust. In dinosaurs, this feature was used as a prey-seizing stroke (Padian et al. 1998).

It would have only taken a slight adjustment of the angle of attack of this predatory stroke to create a suitable vortex wake. By running, leaping and a few such strokes, extension of the time in the air, and eventually flight from the ground up could have evolved. This idea requires no features not already known fromfossils. It has been suggested that advantage may have been taken of any ridge or incline and so the model does have an element of the arboreal theory (Padian et al. 1998).Overall, the evidence seems to point to a modified version of the cursorial theory of bird. Running leaps were aided by wings outstretched for balance, the wings were expanded at the distal ends for increased stability. The leaps were gradually extended by short flapping motions that elaborated the down and forward motion already present in the sister groups of the first birds. Running and leapingmay have been enhanced by ridge-gliding or jumping from small heights (Padian et

al. 1998).The Arboreal TheoryThe arboreal theory is more intuitive in that flight evolving from an arboreal gliding stage would seem to be relatively easy. However, this theory has little support from comparative biology as it requires the ability to climb trees and toglide. Neither capacity seems to be present in Archaeopteryx or in theropod dinosaurs.Archaeopteryx has none of the features of typical vertebrate gliders, nor is itaerodynamically designed to fly. It has been argued that the lateral grooves andcurvature of the claws are an arboreal specialisation, but these have also beencompared to those of ground-dwelling birds. The evidence for an arboreal lifestyle overall is weak. It should also be considered that large theropods such as Allosaurus and Tyrannosaurus have curved claws with deep lateral grooves, and the

y were obviously not arboreal. Also, palaeobotanical evidence shows an absence of large trees anywhere near the Solnhofen lagoons in which Archaeopteryx is preserved (Padian et al. 1998).Flight Capabilities of ArchaeopteryxThe general consensus is that Archaeopteryx was a weak flier. This is supportedby two arguments. The first is that Archaeopteryx lacks evidence of a supracoracoideus system, which in birds is the tendon that powers the upstroke. Experiments have shown that pigeons with a severed supracoracoideus tendon can not take-off from ground level, can not maintain level flight and can not land safely. However, it is questionable whether Archaeopteryx can be compared with modern birds


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in this way (Padian et al. 1998).The second argument is that the feathers of Archaeopteryx are asymmetrical, which suggests that it was capable of some flight. However, data shows that the asymmetry is less than that of modern fliers and gliders and this argument is only valid if it is assumed that fossil animals must look exactly like modern ones when performing the same function (Padian et al. 1998).Generally it is thought that Archaeopteryx could glide as well as most modern birds and fly by flapping to some extent. The evidence for this is:its wing planform and size are like those of some modern weakly flying birdsthe flight feathers are well-developedthe sternum was a strong site of origin for flight musclesits aerodynamic planform is unlike that of birds that only glide (Padian et al.1998).Flight Improvements After ArchaeopteryxPhylogenetic analysis has shown that many of the characteristics associated withthe origin of flight were already present in non-avian theropod dinosaurs before birds evolved. Feathers evolved in non-avian coelurosaurs whose forelimbs weretoo short to bear functional wings. Archaeopteryx had a forelimb longer than the hindlimb, and flight feathers on the wings and long tail feathers. It is thought to have had a fully evolved flight stroke capable of generating thrust as well as lift. The presence of an alula in the early Cretaceous Eoalulavis shows that the wing mechanism that allowed flying at lower speeds and to manoeuvre like living birds evolved early in bird history. Iberomesornis, of the early Cretaceous, has features diagnostic of a perching ability (Padian et al. 1998).

What kinds of isolation can lead to the formation of a new species?IntroductionAccording to the biological species concept, populations are different species if gene flow between them is prevented by biological differences, known as reproductive barriers. If populations exchange genes they are conspecific, i.e.. Belong to the species, even if they differ greatly in morphology. If they are reproductively isolated, they are different species even if they are indistinguishablephenotypically. Therefore speciation arises from the evolution of biological barriers to gene flow (Futuyma, 1998).The factors leading to reproductive isolation can be divided into two categories; prezygotic factors, which operate before fertilisation can occur; and postzygotic factors, which operate after fertilisation leading to partial or complete failure of crosses between the two forms. These are summarised below:

Prezygotic factors

Geographical isolation: Forms are separated by land or water barriers that theyare unable to cross.Ecological isolation: The forms fail to meet because they live in different places within the same geographic region.Temporal isolation: The forms are active at different seasons or times of day.Behavioural isolation: The forms meet, but do not mate.Mechanical isolation: Copulation occurs, but no transfer of male gametes takes place.Gametic incompatibility: Gamete transfer occurs, but egg is not fertilised.Postzygotic factors

Zygote dies: Zygotic mortality soon after fertilisation.F1 hybrids (first generation) have reduced viability (hybrid inviability).F1 hybrids viable but have reduced fertility (hybrid sterility)Hybrid breakdown: Reduced viability or fertility in F2 (second generation) or backcross (F1 crossed with parents) generations.Prezygotic Barriers: One or more of these may be operating within a given population at any time. The evolutionary functions of these mechanisms are the same, to limit or prevent gene flow between species. They may occur only partially; forexample, behavioural isolation can be complete or females may only show a slight preference for males of their own species (Dobzhansky et al. 1977).

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