Mineral Chemistry and Crystallography

Definition of a Mineral All minerals: 1) Occur naturally 2) Are inorganic solids 3) Have a definitive chemical formula 4) Have a crystalline structure

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

Mineral Chemistry 3) 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 (Mg 3 AlSi 3 O 10 (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 Chemistry  Crystal 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 Chemistry  Bond 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 SiO 4 tetrahedra are very common in the Earth's crust and mantle.  Silicates can be sub-classified based on linkages of the SiO 4 tetrahedra.

Mineral Formation Minerals 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 (CaCO 3 ) and Gypsum (CaSO 4 )) 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 Collection 1Quartz11Barite21Galena 2Pyroxene12Gypsum22Chalcopyrite 3Amphibole13Apatite23Pyrrhotite 4Talc14Calcite24Bornite 5Biotite15Garnet25Graphite 6Muscovite16Hematite26Fluorite 7Phlogopite17Magnetite27Halite 8Orthoclase18Corundum28Malachite 9Plagioclase19Sphalerite29Chromite 10Olivine20Pyrite30Sodalite

Chemical Composition of the Earth’s Crust ElementAtom (Ion) Percent (by weight) OxygenO SiliconSi AluminumAl 3+ 8 IronFe 2+ or Fe 3+ 6 MagnesiumMg 2+ 4 CalciumCa PotassiumK SodiumNa 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.

Silicates mineralformulamineralformula Quartz (1) SiO 2 (framework silicate) Orthoclase (8) (Feldspar) (3-D framework structure) KAlSi 3 O 8 Pyroxene (2) CaMgSi 2 O 6 (single chain silicate) Plagioclase (9) (Feldspar) (3-D framework structure) CaAl 2 Si 2 O 8 Amphibole (3) A 2 Z 5 Si 8 O 22 (OH) 2 or (double chain silicate) Olivine (10) (isolated silicon tetrahedrons) (Mg,Fe) 2 SiO 4 Talc (4) Mg 3 Si 4 O 10 (OH) 2 (sheet silicate) Garnet (15) (ring structure) A 3 Z 2 Si 3 O 12 A 2+ B 3+ Biotite (5) mafic mica (sheet silicate) Sodalite (30) Feldpathoid – feldspar like structure Na 8 Al 6 Si 6 O 24 Cl 2 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 blue b) At center, a space filling model c) 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 Si 4+ cation fits almost perfectly in the middle of a tetrahedron formed of larger O 2- 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 Structure Isolated 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 M 2 SiO 4. Typically the M cations are Mg 2+ and Fe 2+. Pure Mg 2 SiO 4 is known as Forsterite and Fe 2 SiO 4 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 Structure Isolated 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 Structure Metal cations Time for a personal plug – In his groundbreaking, epic work “High Temperature Cation Ordering in Nickel Magnesium Olivines”, Mr. Snyder studied how Ni 2+ would preferentially take the M1 location while Mg 2+ 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 Structure Single 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: CaMgSi 2 O 6 Hedenbergite: CaFeSi 2 O 6 Enstatite: MgSiO 3 Wollastonite: CaSiO 3 Since silica tetrahedra share oxygens with adjacent tetrahedra, the Si:O ratio is 1:3 for pyroxenes. Pyroxenes have the formula M 2 SiO 3 or ABSi 2 O 6. M cations are typically Mg, Fe and Ca.

Silicates: Amphibole Structure Double 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: Ca 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2 Actinolite: Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 Amphiboles are more common in metamorphic than igneous rocks.

Silicates: Mica Structure Sheet 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 formula XY 2–3 Z 4 O 10 (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 Fe 3+ or Ti. Common micas include: Biotite: K(Mg,Fe 2+ ) 3 (Al,Fe 3+ )Si 3 O 10 (OH,F) 2 Phlogopite: KMg 3 AlSi 3 O 10 (F,OH) 2 Muscovite: KAl 2 (Si 3 Al)O 10 (OH,F) 2 Talc: Mg 3 Si 4 O 10 (OH) 2 and Chlorite have similar sheet silicate structures. Micas are more common in metamorphic than igneous rocks.

Silicates: Sheet Silicates This 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 Structure Framework Silicate Feldspar has a 3 dimensional silica structure with a Si:O ratio of 3:8. Common feldspars include: Orthoclase: KAlSi 3 O8 Albite: NaAlSi 3 O 8 Anorthite: CaAl 2 Si 2 O8 Albite and Anorthite form a solid solution of feldspars called Plagioclase. Feldspars are the most common mineral in the Earth’s crust!

Silicates: Feldspar Composition Orthoclase: KAlSi 3 O 8 Albite: NaAlSi 3 O 8 Anorthite: CaAl 2 Si 2 O 8 Albite and Anorthite form a solid solution of feldspars called Plagioclase. Feldspars fall in the range seen in the diagram. OrthoclaseAnorthite

Quartz (SiO 2 ) 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 (SiO 2 ) Quartz has a vast variety of colours due to chemical impurities clear – quartzblack - smoky purple – amethystyellow – citrine pink – rose Quartz 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