GEOL 333 #7 - Mineral Classes and Silicate Classification

Classification of Minerals - Certain small cations with large ionic charge (e.g., Si4+, C4+, and S6+) combine readily with oxygen to form molecular groups of anions (radicals), such as SiO44-, CO32-, and SO42-. Minerals are classified by their chemical composition, mostly based on anion (radical) group. Reason = minerals in same class (same anionic group) resemble each other and often occur together in same rock. Minerals can be further separated in groups based on crystal structure (e.g., silicates subdivided into sheet silicates, framework silicates, etc.).

Mineral Classes

1) Silicates (contain SiO44-) - most important rock-forming minerals, comprise >90% of minerals in crust, many subclasses based on different crystal structures, largest class with ~800 minerals.

2) Halides (contain Cl-, Br-, I-, and F-) - salts that form by chemical precipitation (dissolved ions in water combine to form solid minerals)

3) Oxides and Hydroxides (cations bonded to O2- or OH-) - e.g., Al- and Fe-oxides and Al- and Fe-hydroxides (e.g., hematite, corundum)

4) Carbonates (contain carbonate ion, CO32-) - e.g., calcite and dolomite

5) Sulfates (contain sulfate ion, SO42-, i.e., sulfur in oxidized state, 6+) ­ e.g., gypsum

6) Native Elements (pure elements) - includes metals and non-metals (Cu, Au, C ­ diamond), only ~20 minerals

7) Sulfides (contain sulfide ion, S2-, i.e., sulfur in reduced state, 2- or 1-) - contains many economically important minerals, (metallic) cations include Cu, Fe, Zn, Pb, e.g., pyrite, galena

8) Others (Nitrates, Borates, Chromates, Tungstates, Molybdates, Phosphates, Arsenates, Vanadates, and Sulfosalts) - more rare, but also economically important

Silicate Classification - Fundamental building block of silicate minerals = silica tetrahedron, (silicon cation surrounded by four oxygens, arranged in pyramid shape).

 

 

 

 

Si-O bonds are very strong (~50% covalent and ~50% ionic bonds). Chemical formula for individual silica tetrahedron = SiO44-. This basic unit does not exist as mineral in nature. Why?

 

Compared to SiO44-, individual silicate minerals either add cations (> positive charge) or share oxygens (< negative charge). Si-tetrahedra never share >1 oxygen (bridging oxygen) with adjacent tetrahedron. Why?

 

Isolated tetrahedra (no shared oxygens) and vertex (i.e., corner) sharing tetrahedra occur, but not edge or face-sharing tetrahedra (Fig. 13.13). Sharing of bridging oxygens in silicate minerals = polymerization (> oxygen sharing means > polymerization). Six major types (subclasses) of silicates based on degree of polymerization:

Silicate Subclasses

Isolated tetrahedra (Nesosilicates) single tetrahedra with no shared oxygens (cations added), Si:O ratio = 1:4, olivine.

Sorosilicates (less important geologically) pairs of tetrahedra that share 1 oxygen, Si:O ratio = 2:7, epidote.

 

Cyclosilicates (less important geologically) cyclic rings of 6 tetrahedra, share 2 oxygens, Si:O ratio = 1:3, tourmaline.

 

Inosilicates (chain silicates) long chains of linked tetrahedra; two subgroups (a) single chains share 2 oxygens, Si:O ratio = 1:3, pyroxene. (b) double chains share 2.5 oxygens, Si:O ratio = 4:11, amphibole.

 

Sheet (phyllo) Silicates continuous flat sheet (extension of double chain structure in 2 dimensions), share 3 oxygens, Si:O ratio = 2:5, kaolinite and other clay minerals.

Framework (tecto) Silicates 3-dimensional network of shared tetrahedra, share all 4 oxygens, Si:O ratio = 1:2, quartz and feldspar. Quartz = SiO2, electrically neutral, no extra cations added.