How anions and cations fit together control mineral structures •Ionic Substitution : substitution of similar ions in mineralsoSubstitution is possible if ions are of similar size and charge; e.g. Mg2+ and Fe2+oIf the ions are of unlike charge, it requires a coupled substitution; e.g. Na+ + Si4+ = Ca……•Compounds : develop when cations and anions combine in specific ratios e.g. NaCl or CaCl2oMolecule- smallest unit with the properties of the compound•Complex Ions : anionic groups formed from 2 or more different ions act like a single anion; e.g. (CO3)^2- or (SiO4)^4- oCommonly the basis for mineral classification Rock-Forming Minerals •Over 4200 mineral species are known, but only about 30 are common •These 30 make up crustal rock = rock-forming minerals •Reflect abundance of elements in the Earth’s crust •Classify minerals based on chemistry AND structureoChemistry classification based on anion or anionic groupsoPolymorphs – minerals with the same chemical composition (compound) but different structures; e.g. C = diamond and graphite; Al2SiO5 form at different P,T •Chemical Classes of minerals: typically based on type of anion or anionic group oSilicates – anionic building block: SiO4^4-oCarbonates – anionic building block: CO3^2-
Minerals 17:40oOxides – anionic building block: O^2- oSulfides – anionic building block: ~S^2-oSulfates – anionic building block: SO4^2- oPhosphates – anionic building blocks: PO4^3-oHalides – anionic building blocks: Cl-, F-, I-, Br- oHydroxides – anionic building block: OH-oNative element – no anion but all the same element •Formation of Silicate MineralsoThe silicate ion forms the tetrahedron oTetrahedral are the basic building blocks of all silicate minerals. About 95% of the Earth’s minerals are silicates oLinkages of silicate tetrahedral (polymerization) determines class of silicates Isolated tetrahedra – olivine Orthosilicates (neosilicates)Olivine – (Mg, Fe)2SiO4 common mantle mineral, igneous rocks •Silica tetrahedron apex toward you •Silica tetrahedron apex away from you Kyanite, andaluzite, sillimanite = Al2SiO5^- •Polymorphs, common metamorphic mineral Garnet – (Fe, Mg, Mn, Ca…….Single chains – pyroxene Single chain with 90 degree clevage, common igneous mineral
Minerals 17:40Double chains – amphiboleCommon igneous/metamorphic mineral with 60/120* cleavage Sheet - Muscovite White mica – common metamorphic mineral
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