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625-102 GEOLOGY Lecture 3 Silicate Mineralogy Melbourne EarthEarth Sciences Materials Anion groupings ● Minerals are classified according to their chemistry – (ie not symmetry, hardness or any other physical property) ● They are grouped according to their anions ● Why? – because minerals rarely contain more than one of these, whereas they often contain several different cations ● Minerals are classified into 8 major groups Earth Materials Classification of Minerals ● ● ● ● ● ● ● ● Native elements Sulphides Halides Oxides Carbonates Sulfates Phosphates Silicates - no anions - S2- anions, with variations - halogens (Cl, F; rarely Br, I) - O2- anions (& hydroxides) - CO32- etc. polyanionic gps - SO42- groups with variations - PO43- with variations -SiO44- with structural variations Earth Materials Native Elements Native Sulphur, Italy Native Silver, Germany Earth Materials Sulphides Galena, PbS Marcasite, FeS2 Earth Materials Halides and Oxides Fluorite, CaF2 Magnetite, Fe3O4 Earth Materials Carbonates Calcite, CaCO3 Malachite, Cu2[(OH)2/CO3]) Earth Materials Sulphates, Phosphates Gypsum, CaSO4.2H2O Apatite, CaPO4 Earth Materials Silicates Olivine, (Fe,Mg)2SiO4 Quartz, SiO2 Earth Materials Silicate Minerals ● Most abundant minerals in the Earth's crust ● Make up 95% of the Earth’s crust ● Made up of tetrahedral SiO 4 anion groups ● Polymerised into compound groups by sharing adjacent oxygen ions ● Linked by various cations, esp. Fe2+, Mg2+, Ca2+, K+, Na+ ● Aluminium (Al3+ ) can substitute for silicon ions ● Result in complex chemical compositions Earth Materials The Basic Silicate Unit ● ● ● ● ● Unit behaves as a compound anion Cation radius = 0.41 Å, Anion Radius = 1.40 Å Radius ratio = 0.29, therefore 4-fold Coordination Overall charge of - 4 one negative charge for each oxygen - - SiO4 Tetrahedra Earth Materials Arrangements of silicate groups ● ● ● Silicate groups may exist as isolated tetrahedra joined by cations eg Olivine or joined in more complex polymerised groups Tetrahedra may be arranged as – Simple groups – Rings – Single chains – Double chains – Sheets – Frameworks 3-01 Earth Materials Simple SiO4 groups ● Silicate groups are isolated from each other ● May be individual or simple polymerised groups ● Individual SiO4 tetrahedra are linked by cations ● Closely packed, high density structures ● eg Olivine (Mg,Fe)2SiO4 and Zircon ZrSiO4, Beryl 3-02 Earth Materials Single Chain Structures ● Each silicate tetrahedron shares two oxygens ● Tetrahedra joined into a continuous chain ● Chains are linked with each other by cations ● e.g. the Pyroxene group of minerals Diopside: CaMg(Si2O6) Augite: Ca(Mg,Fe)Si2O6 Hypersthene: MgFe(Si2O6) 3-03 Earth Materials Double Chain Structures ● Tetrahedra linked into a double chain Each SiO4 tetrahedron shares 2 or 3 oxygens ● Chains are linked with each other by cations ● e.g. the Amphibole group of minerals ● 3-04 Earth Materials Sheet Silicates 3-05 ● Continuous layer of linked SiO4 groups ● Each SiO4 tetrahedron shares three oxygens ● Layers are stacked and linked together by cations ● Have strong cleavage, generally soft minerals ● e.g. the Mica and Clay minerals etc. Earth Materials Stacking Silicate Sheets ● The tetrahedral layers can be stacked together in several different ways, e.g.: Clays Micas Earth Materials Framework Silicates ● Interlocking framework of SiO4 tetrahedra ● Each SiO4 tetrahedron shares all four oxygens ● Basic formula unit is SiO2 ● e.g.: Quartz SiO2 ● Feldspars (note role of Al substitution): Orthoclase: KAlSi3O8 Albite: NaAlSi3O8 Anorthite: CaAl2Si2O8 3-05 Earth Materials Feldspar Compositions K Potassium Feldspars No Feldspars occur Na Plagioclase Feldspars Ca Earth Materials REFERENCES ● Hamblin & Christiansen, Chapter 3, p.64-72 ● Skinner and Porter, Chapter 3, p. 53-59 ● Clark and Cook, Chapter 6b, p. 116-122 Earth Materials