Mineral GroupsMineral Chemistry and Crystallography

Definition of a MineralAll minerals: 1) Occur naturally 2) Are inorganic solids3) Have a definitive

chemical formula 4) Have a crystalline

structure

Mineral ChemistryAn understanding of minerals require a knowledge of basic chemistry, most specifically chemical bonding. As a result we will review some basic chemistry:

Bonding occurs when atoms and ions share electrons. The 4 basic types of bonding are:1)Ionic bonds - electron(s)

completely transferred - e.g., Halite (NaCl) - tend to be brittle bonds

2)Covalent bonds - electron(s) are more evenly shared - e.g., Diamond (C) - strong bonds (diamond is strongest))

Mineral Chemistry3) Metallic bonds - transition

metal nuclei swimming in a sea of shared electrons - e.g., Silver (Ag) - malleable

4) Intramolecular bonds - including hydrogen bonds and Van der Waals bonds - generally weak (often feels greasy) - e.g., graphite (C) or talc (Mg3AlSi3O10(OH)2) * Since this course does not require that you have any Chemistry courses as a prerequisite , please ask the teacher for help learning some of these basic chemistry ideas if you have not taken Chem. (Good Video)

Crystal ChemistryCrystal Systems - mineral groupings based on internal symmetry - sometimes reflected in ideal crystal shapes - e.g. quartz belongs to the hexagonal crystal system, halite is cubic Unit Cell - smallest building block that has all of a mineral's structural and chemical characteristics. Unit cells fall into one of the 6 crystal systems.

Crystal ChemistryBond types and crystalline structure are directly related to physical properties (see below) - e.g. Diamond is a colourless, very hard, cubic due to its arrangement of carbon atoms. Graphite is a silver, very soft, hexagonal mineral based on its arrangement of carbon atoms.

Diamond:octahedral crystal (Cubic Crystal System)

Graphite:Hexagonal prism(Hexagonal Crystal System)

Crystal Chemistry The majority of the world’s minerals are silicates. 95% of the crust is made up of just 9 minerals – ALL silicates – known as the rock forming minerals (ex. quartz, feldspar, amphiboles, pyroxenes, micas, etc.)Silicate minerals, containing covalently bonded SiO4 tetrahedra are very common in the Earth's crust and mantle. Silicates can be sub-classified based on linkages of the SiO4 tetrahedra.

Mineral FormationMinerals form:1) As magma or lava cools, minerals form inside the

earth through crystallization. (Ex. Most rock forming silicates)

2) When materials dissolved in water crystallize through evaporation. (ex. Halite (NaCl), Calcite (CaCO3) and Gypsum (CaSO4))

3) From solutions heated by magma (ex. Igneous intrusions, the mid-ocean ridge). Under great pressure water can exist at 400°C or greater. Water dissolves minerals, travels through fissures and minerals are deposited as the temperature decreases. Solutions are also heated at great depths in the Earth’s crust (ex. Gold, Quartz, Pyrite)

4) From the chemical and physical alteration due to metamorphism (ex. Garnet or Kyanite)

Mineral Collection1 Quartz 11 Barite 21 Galena

2 Pyroxene 12 Gypsum 22 Chalcopyrite

3 Amphibole 13 Apatite 23 Pyrrhotite

4 Talc 14 Calcite 24 Bornite

5 Biotite 15 Garnet 25 Graphite

6 Muscovite 16 Hematite 26 Fluorite

7 Phlogopite 17 Magnetite 27 Halite

8 Orthoclase 18 Corundum 28 Malachite

9 Plagioclase 19 Sphalerite 29 Chromite

10 Olivine 20 Pyrite 30 Sodalite

Chemical Composition of the Earth’s Crust

Element Atom (Ion) Percent(by weight)

Oxygen O2- 46Silicon Si4+ 28

Aluminum Al3+ 8Iron Fe2+ or Fe3+ 6

Magnesium Mg2+ 4Calcium Ca2+ 2.4

Potassium K1+ 2.3Sodium Na1+ 2.1

All others <1

The abundance of elements in the Earth's crust show that silicon and oxygen dominate. As a result, the vast majority of rocks are silicates. Pretty much all minerals found in igneous and metamorphic rocks are silicates. Since sedimentary rocks are weathered and eroded igneous or metamorphic rocks, they are commonly made of silicate minerals as well.

Silicatesmineral formula mineral formula

Quartz(1)

SiO2(framework silicate)

Orthoclase (8)(Feldspar)

(3-D framework structure)

KAlSi3O8

Pyroxene(2)

CaMgSi2O6(single chain silicate)

Plagioclase (9)(Feldspar)

(3-D framework structure)

CaAl2Si2O8

Amphibole(3)

A2Z5Si8O22(OH)2 or(double chain silicate)

Olivine (10)(isolated silicon tetrahedrons)

(Mg,Fe)2SiO4

Talc(4)

Mg3Si4O10(OH)2

(sheet silicate)Garnet (15)

(ring structure)A3Z2Si3O12

A2+ B3+

Biotite(5)

mafic mica(sheet silicate)

Sodalite (30)Feldpathoid – feldspar like

structure

Na8Al6Si6O24Cl2

Muscovite(6)

felsic mica(sheet silicate)

Phlogopite(7)

intemediate mica(sheet silicate)

Silicate Structural Diagrams

Three ways of drawing the silica tetrahedron:

a) At left, a ball & stick model, showing the silicon cation in orange surrounded by 4 oxygen anions in blueb) At center, a space filling modelc) At right, a geometric shorthand model. This is the model favoured by geologists because of their simplicity.

Since the common rock forming minerals are all silicates it is worthwhile showing how the silicon tetrahedron is formed. The smaller Si4+ cation fits almost perfectly in the middle of a tetrahedron formed of larger O2- anions.

Silicates are network covalent solids that are very stable and have high melting points. Within silicate structures are metal cations – so ionic bonds are also found. The more ionic bonds in the structure, the more easily the mineral is broken down through chemical erosional processes.

Mineralogists and Crystallographers find it easier to display silicate structures by using geometric diagrams.

Silicates: Olivine StructureIsolated Silica Tetrahedrons

The diagram shows Olivine with isolated silica tetrahedra. As a result the Si:O ratio is 1:4. The blue dots represented by M1 and M2 are the locations of the 2+ cations. Olivines have the structural formula M2SiO4. Typically the M cations are Mg2+ and Fe2+. Pure Mg2SiO4 is known as Forsterite and Fe2SiO4 is known as Fayalite. Thus Olivine is a solid solution between the two “end members”. Most olivines are about 90% Mg. Olivine is a common constituent of basalts and is common in the mantle.

Silicates: Olivine StructureIsolated Silica Tetrahedrons

This diagram shows the M sites as octahedra since the cations are surrounded by 6 oxygen atoms. Note that the blue silica terahedra point upwards and the pinks ones point down. The large number of cations are the reason that olivine erodes easily into other minerals.

Look at Mr. Snyder’s Olivine model to see the hexagonal closest packing of oxygen atoms.

Silicates: Olivine StructureMetal cations

Time for a personal plug – In his groundbreaking, epic work “High Temperature Cation Ordering in Nickel Magnesium Olivines”, Mr. Snyder studied how Ni2+ would preferentially take the M1 location while Mg2+ would choose the M2 location. This “ordering” would decrease as temperature increased. I have a copy of my masters thesis for anybody who is exceptionally bored and wishes to read it.

Silicates: Pyroxene StructureSingle Chain Silicate

Like Olivines, pyroxenes represent a family of minerals. Pyroxenes are common in mafic rocks and are very common in the mantle. Here are a few common pyroxenes:

Diopside: CaMgSi2O6 Hedenbergite: CaFeSi2O6 Enstatite: MgSiO3 Wollastonite: CaSiO3

Since silica tetrahedra share oxygens with adjacent tetrahedra, the Si:O ratio is 1:3 for pyroxenes.

Pyroxenes have the formula M2SiO3 or ABSi2O6 . M cations are typically Mg, Fe and Ca.

Silicates: Amphibole StructureDouble Chain Silicate

Amphiboles have very complicated structures (as seen in the diagram below), but it is simplest to understand that they are double chain silicates (see top diagram). Amphiboles have a Si:O ratio of 8:22.

Common amphiboles include:

Hornblende: Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

Actinolite: Ca2(Mg,Fe)5Si8O22(OH)2

Amphiboles are more common in metamorphic than igneous rocks.

Silicates: Mica StructureSheet Silicate Micas have 2-dimensional sheets of

silica tetrahedra. Layers of these silica tetrahedra sheets are separated by hydroxyl ions. (see top diagram). Sheet silicates have a Si:O ratio of 2:5.

Chemically, micas can be given the general formulaXY2–3Z4O10(OH,F)2 in which:

X is K, Na, or Ca or less commonly Ba, Rb, or Cs;Y is Al, Mg or Fe or less commonly Mn, Cr, Ti, Li;Z is Si or Al but also may include Fe3+ or Ti.

Common micas include:Biotite: K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2

Phlogopite: KMg3AlSi3O10(F,OH)2

Muscovite: KAl2(Si3Al)O10(OH,F)2

Talc: Mg3Si4O10(OH)2 and Chlorite have similar sheet silicate structures.

Micas are more common in metamorphic than igneous rocks.

Silicates: Sheet SilicatesThis diagram shows how sheet silicates look in side view. Note the silica tetrahedra in light blue and the Mg octahedra in dark blue.

In between the sheets of silica tetrahedra are hydroxyl (OH-) ions.

The covalent bonds in the mica sheets are very strong but the ionic bonds formed by the hydroxyl ions are very weak.

Hence the bonds between layers of mica are very weak which explains why layers of mica can be peeled apart.

Silicates: Feldspar StructureFramework Silicate

Feldspar has a 3 dimensional silica structure with a Si:O ratio of 3:8.

Common feldspars include:

Orthoclase: KAlSi3O8Albite: NaAlSi3O8

Anorthite: CaAl2Si2O8

Albite and Anorthite form a solid solution of feldspars called Plagioclase.

Feldspars are the most common mineral in the

Earth’s crust!

Silicates: Feldspar CompositionOrthoclase: KAlSi3O8

Albite: NaAlSi3O8

Anorthite: CaAl2Si2O8

Albite and Anorthite form a solid solution of feldspars called Plagioclase. Feldspars fall in the range seen in the diagram.

Orthoclase

Anorthite

Quartz (SiO2)Quartz is composed of silica

tetrahedra linked at their corners in a hexagonal crystal structure

Quartz (SiO2) is very common in igneous (especially felsic) and metamorphic rock. Sandstone is essentially pure quartz.

When silicate mineral erode (weather), they break down chemically. Ionic bonds break first and metallic minerals are removed and oxidized leaving the tough silica tetrahedra (Quartz)(which are held together with covalent bonds).

Quartz (SiO2)

Quartz has a vast variety of colours due to chemical impurities

clear – quartz black - smokypurple – amethyst yellow – citrinepink – roseQuartz is often deposited by hot

solutions into hard, layered rocks that are often very colourful (agate, chalcedony, red jasper, black flint)

These quartz solutions often form veins that host a variety of important minerals – gold, copper, molybdenum

Other Silicates:Garnets, Tourmalines, Ring Silicates