MINERALS

MINERALS
takes place deformation
Minerals are naturally occurring, inorganic substances, usually possessing a definite chemical composition and a characteristic atomic structure. About 2000 minerals are known to be existing. Rocks are composed of various combinations of minerals. Some minerals form a series in which there is a gradual replacement of one element by another. The most common mineral in rocks is quartz. Silica often combines with other oxides to form silicates, the most common of which are feldspars. Mica is a common silicate. Feldspars are silicates of aluminium, potassium, sodium and calcium. Clay minerals are complex silicates derived from weathered minerals. The term mineral is also used for any naturally occurring material which is mined and is of economic value.

PHYSICAL PROPERTIES Crystal structure, cleavage and fracture, specific gravity, hardness, lustre, colour and streak are important aspects for understanding physical properties of rocks.

Crystal Structure Mineral crystals fall into six catego­ries each of which is defined in terms of its crystallographic axes: (i) isomeric system (halite mineral); (ii) tetragonal system (zircon material); (iii) hexagonal system (beryl mineral); (iv) orthorhombic system (staurolite mineral); (v) monoclinic sys­tem (gypsum mineral); and (vi) triclinic system (albite mineral).

Specific Gravity Specific gravity of minerals deter­mines the density of a given rock and rock density in turn determines the gross layered structure of the earth.

Hardness It determines how easily a mineral is worn away by the abrasive action of streams, waves, wind and glaciers in the processes of erosion and transportation. Ten standard minerals constitute the Mohs Scale of hardness ranging from the softest to the hardest.

Lustre The appearance of a mineral surface under reflected light is referred to as its mineral lustre. It is described by several descriptive adjectives, such as metallic (metal like), adamantine (diamond like), vitreous (glass like), resinous (oil like), pearly or silky (pearl or silk like).

Colour Certain minerals possess a distinctive mineral colour that facilitates recognition. The impurities present in the mineral provides shades of colour to a mineral.

Streak When a mineral is rubbed across the white porceline plate, known as streak plate, it may leave a streak of mineral powder of distinctive colour. The colour of powder of mineral may sometimes be different than the colour of the mineral.
(Note: Regional distribution and economic importance of minerals are dealt separately in different chapters.)