Principal Stone forming Minerals

About 200 minerals have been recognized by the chemists and geologists but a few of them have got engineering application. The following are the important rock forming minerals.
1. Silica: Quartz is a pure Silica. It is undaffected by weathering. Its colour may be white, grey, pink, purple or yellowish. Streak is white. No cleavage, luster is vitreous and the structure is granular. It is hard and specific gravity is 2.66. It is used in the manufacture of bricks, ceramics, glass, concrete, mortar, plaster, etc. 
2. Feldspars: Contains Silica, Alumina, Potassium, Sodium or Calcium, Generally two types:
(a) Potash Feldspars: Also known as orthoclause. It contains Potassium, Alumina and Silica. Its colour is white, reddish, grey, or grass. Streak is white 2 cleavage at 90 degree, luster is vitreous, granular in structure, very hard, sp. Gravity is 2.5 to 1.7. It is used in the manufacture of porcelain wares, glasses and also for glazing.
(b) Lime-Soda Feldspars: Also known as plagioclause. It contains Sodium, Calcium Alumina and Silica. Its colour is generally white, may be gray, blush or reddish. Streak is white 2 cleavage at 90 degree. Luster is vitreous. Structure is qranular or fibrous, very hard, sp. Gravity is 2.4 to 2.85. It is used for manufacturing glasses, and ceramic products.
3. Micas: Complex Silicates of Potassium, Alumina, Silica, Magnesium and Iron. It is generally of two types:

(a) Muscovite: Complex silicate of Potash, and Alumina. Its colour is generally white, yellowish or grey, Steak is white, luster is vitreous, structure foliated very weak, one perfect cleavage, sp.gr=3.0. It disintegrates very rapidly, hence a source of weakness is present in building stones.

(b) Biotite: It is a complex silicate of Potassium, Magnesium, Iron and Alumina. Its colour is black, one perfect cleavage, luster is vitreous. Structure is foliated, moderately hard, sp. Gr=3.6.

4. Amphibole: Also known as Hornblende. This is a complex silicate of Sodium, Calcium, Magnesium, Iron and Aluminium, with a dark green to black colour. Streak is pale black, two cleavage at 90 degree. Luster is vitreous, Structure is granular and very hard. sp. Gr=3.2.

5. Pyroxene: Also known as Augite. It is a complex silicate of Calcium, Magnesium, Iron and Aluminium. It is similar to hornblende, but slightly harder and heavier. Its colour is green, streak is greenish cleavage 2 at 90 degree, luster is vitreous, structure is granular.

6. Olivine: It is composed of Magnesium, Iron and Silica. Yellowish green colour, streak is pale green, Cleavage is not distinct, Luster is vitreous, structure is granular, very hard. Sp. Gr. Is 3.5.