Ionic radius is related to the valence of the ion - ions that have lost electrons (cations) are smaller than their neutral state, ions that have gained electrons (anions) are larger.

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If two cations are similar in ionic radius, one may substitute for another in a mineral structure. One of the more common substitutions is between Fe2+ (~0.63 angstrom) and Mg2+ (~0.57) and often a mineral contains a mixture of both

Example is olivine - Fe2SiO4 = fayalite; Mg2SiO4 = forsterite; olivine = (Mg,Fe)2SiO4

Ca2+ and Na+ can also substitute for each other in plagioclase feldspar.

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Silicates (SiO4)4-: The largest mineral group due to the fact that silicates are made up of the two most abundant elements in the crust: silicon and oxygen.

The basic building block is the silica tetrahedra

As oxygen : silicon ratio decreases, more and more silicon ions must share oxygen atoms in order to complete their tetrahedra.

Silicate mineralogy

1.Isolated tetrahedra (monomer). O:Si = 4, olivine is the most common example (Fe,Mg)2SiO4 no cleavage. Understand Solid solution of Fe-Mg

2.Single chain - silicate tetrahedra share oxygens with two other tetrahedra forming a long open ended chain. O:Si = 3 , pyroxenes, 2 cleavages at ~90, Si2O6

3.Double chain: two singles chains link together so each tetrahedra shares oxygens with three other tetrahedra. O:Si = 2.75, amphiboles, 2 cleavages at 60 and 120, Si8O22

4.Sheet silicates: silica tetrahedra form large sheets (link to three other tetrahedra) with all of the non-shared oxygens pointing in the same direction, Micas (biotite, muscovite):1 direction of cleavage. O:(Si+Al) = 2.5

5. Framework silicates: 3-d frameworks, all oxygens shared. O:Si+Al = 2. Common groups include quartz and feldspars (potassium feldspar/orthoclase, plagioclase). Most abundant minerals in the earth’s crust. (Solid solution in plagioclase)

Can see increasing amount of SiO2 with increasing polymerization-also lower temperature.

• Cabonates: calcite, dolomite

• Oxides: magnetite, hematite

• Sulfides, sulfates, halides, native elements

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How to ID minerals:

Hardness: Moh’s scale

Specific gravity

Other properties (fizz, magnetic)

Moh’s scale

Color not always!

streak

Crystal habit

Cleavage-feldspars

Cleavage

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Concoidal fracture

Carbonates fizz