ABSTRACT
This chapter illustrates only a few minerals which are very common and which are necessary for the identification of the majority of igneous and sedimentary rocks. Olivine is a common constituent of basic and ultrabasic igneous rocks where its composition is magnesium rich: it is usually accompanied by a clinopyroxene which has a brownish colour, whereas the olivine is almost colourless or slightly greenish in colour compared with the pyroxene. Chemically the commonest clinopyroxenes differ from orthopyroxenes in that the former contain essential calcium. The amphibole group of minerals contains a large number of different solid solutions but all of them have similar crystal structures despite the great variety of chemical substitutions which are possible. Two varieties of mica are common in rocks; colourless muscovite and brown biotite. The birefringence of chlorite is much less than that of the micas, and chlorites commonly show anomalous interference colours which do not appear in the interference chart.
