ABSTRACT
Minerals are the solid constituents of all rocks, igneous, sedimentary and metamorphic, and occur as crystals. The average composition of crustal rocks has been calculated from many chemical analyses. Since silicon and oxygen preponderate in the rocks, the chief rock-forming minerals are silicates. Some minerals have a distinctive colour, for example the green colour of chlorite, but most naturally occurring minerals contain traces of substances which modify their colour. Thus quartz, which is colourless when pure, may be white, grey, pink or yellow, when certain chemical impurities or included particles are present. Many minerals possess a tendency to split easily in certain regular directions, and yield smooth plane surfaces called cleavage planes when thus broken. The separation of fragments of a particular mineral from a sediment containing many minerals may be achieved by placing them in a liquid solution of known specific gravity equal to that of the mineral to be separated.
