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Classification of Minerals Classified on the basis of anionic groups Minerals in each group have grossly similar crystallographic properties and tend to occur in the same geological settings / environments - silicates – igneous and metamorphic rocks - carbonates – sedimentary rocks (also some igneous) - sulfides – hydrothermal systems (fluid–rock dominated) Mineral Classes 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) Native Elements Sulfides + Sulfosalts (As, Sb) Oxides Hydroxides Halides Carbonates Nitrates Borates Phosphates Sulfates Tungstates-Molybdates Silicates S2O2OHCl-, F-, BrCO32NO3BxOy(OH)z PO45SO42WO42- MoO42SiO44- Mineral Class Family Group Species Varieties Silicates 27% of all known minerals and about 40% of common rock forming minerals are silicates. Silicates make up ~ 90% of rocks in the crust of the Earth The more important rock-forming silicate minerals include: olivine, garnet, pyroxenes, amphiboles, micas, clay minerals, feldspars and quartz (Plagioclase is the most abundant mineral in crust). Other important non-silicate rock-forming minerals include calcite and dolomite. Important in Igneous and Metamorphic Rocks and sediments derived from them Important elements are: Si 4+; Al3+; Fe2+; Fe3+; Mg2+; Ca2+; Na+; K+; Ti4+; Cr3+; Mn2+ Radius of Si 4+:O2- 0.26 Å Si-O bond is ~ 50% ionic and ~50% covalent SiO4 has a net – 4 charge such that each oxygen has a potential to bond to another Si, thereby forming links between SiO4 tetrahedra Silicates are classified on the basis of how the silica tetrahedral are linked Al3+ is very important in silicates because it is the 2nd most abundant metal and the 3rd most abundant element. Ionic Radius of Al3+ = 0.39 Å and radius ratio of Al3+:O2- 0.286 Al3+ substitutes for Si4+ in tetrahedral sites Because radius ratio is close to octahedral co-ordination, Al3+ can also go into the 6 coordinated site (octahedral) Substitution of Al3+ into tetrahedral sites allows additional elements Mg2+; Fe2+; Fe3+; Mn2+; Cr3+; Ti4+ to be put in octahedral sites (6-fold co-ordination) Ca2+ and Na+ go into 8-fold (cubic) sites and K+; Rb+ and Ba2+ go into 8 to 12 coordinated sites tetrahedral co-ordinated General Formula for Silicate Minerals Xm Yn (ZpOq) Wr 8-Fold 6-Fold 4-Fold (All formulas must be charge balanced) OH-, Cl-, F- (1) Nesosilicates (SiO4)-4 - independent tetrahedral linked by octahedral sites O:Si = 4:1 (2) Sorosilicates (Si2O7)-6 - two tetrahedral linked by a single oxygen atom O:Si = 3.5:1 (3) Cyclosilicates (Si6O18)-12 or (Si3O9)-6 - rings O:Si = 3:1 (4) Inosilicate (a) Single Chain (SiO3)-2 - O:Si = 3:1 (b) Double Chain (Si4O11)-6 - O:Si = 2.75:1 (5) Phyllosilicates (Si2O5)-2 (sheet silicates) - rings of tetrahedral linked together O:Si = 2.5:1 (6) Tectosilicates (SiO2)0 (3-D Framework) - O:Si = 2:1