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) Native Elements 2) Sulfides + Sulfosalts (As, Sb) S2- 3) Oxides O2- 4) Hydroxides OH- 5) Halides Cl-, F-, Br- 6) 2- Carbonates CO3 7) - Nitrates NO3 8) Borates BxOy(OH)z 9) 5- Phosphates PO4 10) 2- Sulfates SO4 11) 2- 2- Tungstates-Molybdates WO4 MoO4 12) 4- Silicates SiO4

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  tetrahedral co-ordinated

 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 co- ordinated 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 co-ordinated sites  General Formula for Silicate Minerals

Xm Yn (ZpOq) Wr (All formulas must be charge balanced)

8-Fold 6-Fold 4-Fold OH-, Cl-, F-

(1) -4 Nesosilicates (SiO4)

- independent tetrahedral linked by octahedral sites - O:Si = 4:1

(2) -6 Sorosilicates (Si2O7)

- two tetrahedral linked by a single oxygen atom - O:Si = 3.5:1

(3) -12 -6 Cyclosilicates (Si6O18) or (Si3O9)

- rings - O:Si = 3:1 (4) -2 Inosilicate (a) Single Chain (SiO3)

- O:Si = 3:1

-6 (b) Double Chain (Si4O11)

- O:Si = 2.75:1 (5) -2 Phyllosilicates (Si2O5) (sheet silicates)

- rings of tetrahedral linked together - O:Si = 2.5:1

(6) 0 Tectosilicates (SiO2) (3-D Framework)

- O:Si = 2:1