Silicates - Anhydrous Silicates, Disilicates, Polysilicates and The Feldspar Group
The silicates form the largest single section of the Chemical Classification of Minerals. They may be divided into (1) Anhydrous Silicates, (2) Hydrous Silicates.
Anhydrous Silicates
This section may be subdivided into (1) Disilicates, Polysilicates, Being Salts of disilicic acid, H2Si2O5, or polysilic acid H4Si3O8; (2) Metasilicates, being salts of metasilicic acid, H2SiO3;(3) Orthosilicates, being salts of orthosilicic acid, H4SiO4; Sub-silicates, including various basic species.
Disilicates, Polysilicates
The only representative of the disilicates of sufficient importance to warrant mention here is the rare lithium mineral, petalite, LiAl(Si2O5)2
The Feldspar Group
The feldspars form one of the most important of mineral groups. They are polysilicates of aluminum with either potassium, sodium and calcium and rarely barium. The may belong to either the monoclinic or the triclinic systems but with the crystals of the different species resembling each other closely in angles, habits of crystallization, and methods of twinning. They all show cleavages in two directions which make an angle of 90°, wit each other. Hardness is about 6 and specific gravity 2.6.
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