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Geology 229Engineering Geology

Lecture 3

Basic Rock Classification andEngineering Considerations

(West, Chs. 2, 3, 4, 5)

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Outline of this Lecture

1. Rock types and rock cycle2. Geological and engineering definitions of rocks

3. Basic Mineralogy

4. Engineering Geology considerations

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Geology is the study of the earth, from different perspectives,

it studies

Composition Mineralogy, petrology

Surface expression Geomorphology

Structure Structural geology

Internal activities Global geophysicsFormation process Stratigraphy, geochronology

Ancient Life - Paleontology

The physical nature of the earth and its interaction with

engineering construction - Engineering Geology.

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What is a rock?

In Geology, Rock is defined as the solid materialforming the outer rocky shell or crust of the earth.

There are three major groups of rocks by its origin:

(1) Igneous rocks: cooled from a molten state;

e.g., granite, basalt ;

(2) Sedimentary rocks: deposited from fluid medium;the products of weathering of other rocks in water;

e.g., sandstone, mudstone;

(3) Metamorphic rocks: formed from pre-existing rocksby the action of heat and pressure.

e.g., dolomite, marble ;

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Rock Cycles

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Plate Tectonic Settings of Volcanoes and Igneous Rocks

Basalt

Gabbro

Andesite

Diorite

e

Andesite

Diorite

Rhyolite

Granite

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Thus, in pure geological sense rock is defined as theessential part of the earths crust. Geologists

concern about the origin, classification, history, and

the spatial aspects of rocks. So, geologicallyspeaking, ice, sand, marble, coal, basalt, can be

simply regarded as rocks. However, the Engineering

Geologists have a different, and relatively narrowerview of rocks.

The Engineering Definit ion of Rocks

Rock is the hard and durable material.

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Basic Mineralogy of Rocks

Rocks are formed with minerals. What is a mineral?

1) a naturally occurring chemical element or compound;

2) formed by inorganic processes;

3) with an ordered arrangement or pattern for its atoms

crystalline structure;4) possesses a definite chemical composition or range of

compositions.

The opposite of mineral property is amorphous, i.e., the

property of non-crystal, order-less property possessed by

glass, volcanic glass, etc.; oil or coal can neither be

regarded as minerals by their organic involvement.

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Basic Mineralogy of Rocks (cont.)

So we can simply express the mineral as

mineral = composition + crystalline structure

For two minerals if the composition are the same but the

structures are different, they can be called a pair of

polymorphs. The common examples for polymorphs include

1) pyrite/marcasite (FeS2 , isotropic vs anisotropic iron

atom spacing);

2) diamond/graphite (C, the same composition ofcarbon but different structure);

3) Calcite/aragonite (CaCO3);

4) quartz/cristobolite (SiO2).

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Mineralogy Identification for Engineering Purposes

From an engineering point of view, certain properties of

minerals, especially when they are introduced into or

encountered with another mineral, are of special concern to

engineers. For example,

gypsum in a limestone can become swelling when

water presents;pyrite (the fools gold) in shale can be deteriorated by

acid water;

swelling clays in shale can become wetting and causeinstability problem of a slope.

Thus, fundamental mineralogical acknowledge is needed

when identifying engineering material is needed.

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Mineralogy Identification

Minerals can be identified by its

color;

streak (strip);

luster;

hardness;

specific weight;

cleavage;fracture;

crystal form;

magnetism;tenacity;

diaphaneity;

striation;

chemical reaction.

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Color

Minerals are colored because certain wave lengths of light are absorbed,

and the color results from a combination of those wave lengths that reach

the eye. Some minerals show different colors along different crystallographic axes.

Mineral identification by colors can be deceptive!!

Flourite

Flourite Flourite

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Streak

The streak of a mineral is the color of the powder left on a streak plate

(piece of unglazed porcelain) when the mineral is scraped across it

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Luster Lusterrefers to how light is reflected from the surface of a mineral. The two main

types of luster are metallic and nonmetallic.

Types of nonmetallic luster

adamantine, vitreous, pearly, greasy, silky, earthy

Apophyllite-pearlyRosequartz - greasy

Quartz - vitreous

Cuprite - adamantineBauxite - Earthy

Chrysolite -silky

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Hardness

Minerals Level of

Hardness

Tools

Talc 1Gypsum 2 Finger nail

Calcite 3

Fluorite 4 Copper penny

Apatite 5

Orthoclase 6

Quartz 7 Glass

Topaz 8

Corundum 9

Diamond 10

The hardness of a

mineral is itsscratchability,

determined by Mohs

hardness scale. Thehardest mineral

known, diamond,

was assigned thenumber 10.

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Specific Gravity

Specific gravity is the "heaviness" of a mineral. Itis defined as a number that expresses the ratioof the weight of a mineral and the weight of anequal volume of water.

The specific gravity depends on:

the kind of atoms that comprise the mineral

how the atoms are packed together

Common rock-forming minerals (quartz,feldspar, calcite, etc.) have specific gravity near2.7

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Cleavage

Cleavage is the ability of a mineral to break along preferred planes

Minerals tend to break along certain planes where atomic bonds are weak

Minerals can have one, two plane or three plane cleavages.

Copper - none

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Crystal forms

Crystal forms are displays of well-formed crystal faces by a mineral

Crystal faces formed during crystallization process vs. cleavage faces

formed when mineral breaks.

Beryl -hexagonal

-

Diamond- octahedron

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Basic Mineralogy of Rocks

There are more than 2000 naturally occurred minerals have

been discovered; only a bit more than 100 are common and

used in college mineralogy. However, of the 100 common

minerals only about 25 are abundant rock-forming minerals.

The main types of minerals are:

metallic minerals;

nonmetallic minerals;

carbonate minerals;

sulfate minerals;

sulfide minerals;silicate minerals;

oxide minerals;

clay minerals.

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Sulfide Minerals

Pyrite: Fools gold, minor ore of sulfur for sulfuric acid, causes staining on

surface of concrete due to oxidation or presence of sulfate ions.

Molybdenite: Nearly 50% of all molybdenum is used in making steel.

Sphalerite: The most important ore mineral of zinc which is used to make brass,electric batteries, and zinc white.

Pyrite

Sphalerite

Molybdenite

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Oxide Minerals

Hematite

Chromite

Ilmenite

Hematite:

Fe2O3- causes staining and popouts

on the concrete surface

Chromite:

-resistant to the altering

affects of high temperatures

and pressures

- component in the bricksand linings of blast furnaces

- major constituent in

stainless steel

Ilmenite:

- major ore of titanium

(aluminum-like metal; light

weight, non-corrosive, able

to withstand temperatureextreme, has many

applications in high tech

airplanes, missiles, space

vehicles)

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Silicates

Most important of all mineral classes because:

- 25% of the known minerals and nearly 40%of the common ones are silicates

- Nearly 90% of the igneous rock-forming

minerals are silicates, which means that they make

up over 90% of the Earth's crust

- Bricks, stones, concrete and glass are either

silicates or derived from silicates

Important silicate groups: Ferromagnesians,

nonferromganesians, feldspar (orthoclase,

plagioclase), Quartz

Silicon-Oxygen Tetrahedron

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Ferromagnesians

Contain Fe or Mg Olivines: Olivine: (Mg, Fe)2SiO4

- found mostly in igneous rock

- olivines variety, peridot, has same chemical

composition as molten magma in Earths mantle. Thus, peridotis considered the most common mineral by volume in the Earth

- industrial uses as refractory sands and abrasives,

an ore of magnesium

Pyroxenes: Augite

Amphibole: Hornblende

Micas: biotite

Olivine

Augite

Biotite Hornblende

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Nonferromagnesians

Contain Ca, K, Na

Soft, flaky, platy, one prominent cleavage minerals

Serpentine: many industrial applications, including brake linings andfireproof fabrics and as an ornamental stone.

Muscovite: used in heat and electrical insulator for industrial purposes

serpentine

Muscovite

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Feldspars (Si3O8 ) By compositions, feldspars is the most common rock-forming silicates

Orthoclase: - contains K

- used in porcelain industry

Plagioclase: - contains Ca, Na- Industrially important in glass and ceramic industries; pottery and

enamelware; soaps; abrasives; bond for abrasive wheels; cements and concretes;

insulating compositions; fertilizer; poultry grit; tarred roofing materials; and as a

sizing (or filler) in textiles and paper.

Orthoclasealbite

O )

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Quartz Minerals (SiO2group)

Second common rock-forming

mineral

silica for glass, electricalcomponents, optical lenses,

abrasives, ornamental stone,

building stone, etc.

Chert: - variety of Quartz,

- found in sedimentary rock

- when used as an

aggregate material, it

easily breaks and pop out

when exposed to freezing

and thawing. Thus, it

reduces the strength of

concrete.

Amethyst quartz

Rock crystal

Rose

smoky

Rock Crystal

C b t i l (CO )

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Carbonate minerals (CO3 group)

Calcite: - fizzes with acid- Primary component in cave

formation, react with carbon dioxide

in sea and air, thus, acts like carbon

dioxide filter for the planet- used in cements and

mortars, production of lime,

limestone is used in the steel industry;

glass industry, ornamental stone,

chemical and optical use

Aragonite: minor constituent of

limestone which is used in cement

and in steel production, ornamental

carvings

Dolomite: Dolomite problem

calcite

Dolomite

Aragonite

Dolomite

Sulfate Minerals (SO group)

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Sulfate Minerals (SO4 group)

Gypsum: -common in sedimentary rock in high saline water.- used in plaster, wall board, some cements, fertilizer, paint filler,

ornamental stone

Anhydrite: - water-free form of gypsum

- in the manufacture of some cement, a source of sulfate for

sulfuric acid

- causes cracks in structure due to property of swelling when wet

and converting to gypsum

Gypsum

Anhydrite

Gypsum

Anhydrite

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Clay Minerals

Very fine-grained minerals, common in soil

Clay = kaolinite, halloysite, illite (non swelling clays), vermiculite, smectite

(swelling clays) Smectite: used in drilling mud since it has property of swelling when

exposed to water

Kaolinite: made up high-grade clay, used in manufacture of ceramic

products, rubber industry, refractories Illite: chief constituent in shales

Kaoliniteillite

Vermiculite

Halloysite

vermiculite

Smectite

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Rock Identification

Rocks are identified mostly by its

texture;

mineral composition;

field relationships;

color;

hardness;specific weight;

crystal form;

magnetism;

Apparently, some techniques used in identifying minerals

can also be used to classify the rock type.

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Rock Properties for Engineering

Rock are significant for two major reasons in

engineering:

(1) As building materials for constructions;

(2) As foundations on which the constructions are

setting;

For the consideration of rocks as construction material

the engineers concern about:

(a) Density to some extent (for calculating theweight, load to the foundation, etc.);

(b) Strength;

(c) Durability;

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For the consideration of rocks as the construction

foundation the geological engineers concern aboutthe rocks:

(a) Density;

(b) Strength;

(c) Compressibility;

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Engineering concerns of different rocks:

(a) Igneous;

(b) Sedimentary;

(c) Metamorphic;

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The Identification Chart of the Igneous Rocks

Engineering Considerations of Igneous Rocks

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Engineering Considerations of Igneous Rocks

(1)Fine-grained igneous rocks cannot be used as aggregates in

Portland cement due to volume expansion caused by the Alkali-silica reaction. Solutions include:

(a)Can be used in low alkali cement;

(b)Non-reactive aggregates go with the high alkali cement;

(c) Add pozzolans, coal-ashes, etc. in the aggregate-cement

mixture to minimize the reaction.

(2)Coarse-grained igneous rocks (e.g., granite, syenite, etc.) are notfor aggregates for constructions because its low abrasion

resistance; but fine-grained igneous rocks (e.g., basalt) are good

fro aggregates (e.g., basalt as paving aggregates goes with

asphalt.

(3)Siting of foundations needs to avoid weathered rocks (e.g., dams,

bridge piers, etc.);

(4) Igneous rocks are good for dimension stone (tombstone etc.)

because their resistance to weathering but need avoid fractures.

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The Nathan Hale

Monument in Coventry,

Connecticut. Built in1851 with granite blocks

quarried from Quincy,

MA.

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Sediments and Sedimentary Rocks

Sediments are pieces of loose debris that have not been lithified.

Sediments are soil, with the engineering definition. Sediments are

the combination of gravels, sands, silts, and clays;

Sedimentary rocks are lithified sediments that held together by

various types of cementing agents, such as calcite, quarts, andiron oxide; or by compaction of the mineral grains into an indurate

mass.

There are three ways in lithification:

compaction - reduction of pore size;

cementation - pores filling by binding agents;

crystallization - new minerals crystallized in pores.

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Sedimentary Rocks

(1)95% of the volume of the crust (the first 10 miles in depth of the

earth) are igneous and metamorphic rocks;

(2)However, it is about 75% of the surface of the earth are covered by

sedimentary rocks;

(3)Consequently, engineers are most likely end up with working more

often on sedimentary rocks.

Sedimentary rocks composed of mineral grains or crystals that

have been deposited in a fluid medium, and subsequently lithifiedto form rocks. Of the sedimentary rocks on the earths surface,

46% are shale, 32% are sandstone and 22% are limestone.

Conglomerate is a clastic sedimentary rock that forms

from the cementing of rounded cobble and pebble sized

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from the cementing of rounded cobble and pebble sized

rock fragments. Conglomerate is formed by river

movement or ocean wave action. The cementing agentsthat fill the spaces to form the solid rock conglomerate

are silica, calcite, or iron oxides.

Chert is a very hard sedimentary rock that is usually

found in nodules in limestone. Chert is light gray to dark

gray in color. It probably formed from the remains of

ancient sea sponges or other ocean animals that have

been fossilized. Silica has replaced the tissue forming the

sedimentary rock.

Limestone is the most abundant of the non-clasticsedimentary rocks. Limestone is produced from the

mineral calcite (calcium carbonate) and sediment. The

main source of limestone is the limy ooze formed in the

ocean.Sandstone is a clastic sedimentary rock that forms from

the cementing together of sand sized grains forming a

solid rock. Quartz is the most abundant mineral that

forms sandstone. Calcium carbonate, silica, or iron has

been added to the water that is in contact with the sandgrains.

Engineering Considerations of Sedimentary Rocks

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(1)The sedimentary rocks also have the Alkali-silica reaction problem

when used as aggregates with Portland cement. The sedimentary

rocks with this problem are chert and graywacke.

(2)Fine-grained sedimentary rocks like limestone and dolomite are thebest for being used as aggregates; siltstone, shale, conglomerate,

and quartz sandstone are not acceptable;

(3)Stream and terrace gravel contains weak pieces, they are not good

for aggregates in concrete. Weathered chert, shale, and siltstone

can cause pop-outs at the concrete surface after freeze-thaw

cycles;

(4)Coarse-grained limestone is not good for aggregates by reducingparticle size;

(5) Sinkhole problem in carbonate terrains due to the high

dissolvability of limestone and dolomite.

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Engineering Considerations of Metamorphic Rocks

(1)The metamorphic rocks also have the Alkali-silica reaction problem

when used as aggregates with Portland cement. The metamorphic

rocks with this problem are argillite, phyllite, impure quartzite, andgranite gneiss;

(2)Coarse-grained gneiss can be abraded severely when used as

aggregates;(3) For metamorphic rocks the stability of rock mass greatly affected

by the foliation orientation;

(4)Marble as a metamorphic rock from carbonate sedimentary rockscan cause similar problems, eg., leakage of reservoirs, sinkhile

collapse, solution cavities, and channels.

f

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reference

http://www.theimage.com/mineral/index.ht

m http://webmineral.com/specimensM.shtml

http://www.theimage.com/mineral/index.htmhttp://www.theimage.com/mineral/index.htmhttp://www.theimage.com/mineral/index.htmhttp://www.theimage.com/mineral/index.htm

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Readings:

Ch. 2, 3, 4, 5

Homework:

Chapter 3: Problem 9

Chapter 4: Problem 6