ABSTRACT
Igneous rocks are formed by the solidification of magma. The minerals present in a rock and the relative proportion of minerals (mode) is largely dependent on the chemical composition of the magma. Mineralogy and grain size is the basis of most classification. In simplified classification, often most useful to students, igneous rocks are separated on their mineralogy, specifically the presence or absence of quartz, the type and relative proportion of feldspar present (alkali feldspar and plagioclase, and composition of plagioclase), and the type of iron or magnesium (ferromagnesium) minerals present, combined with the grain size. An example are those rocks that can be grouped under the term lamprophyre. This group of rocks occurs as dykes and are characterized by having phenocrysts of a ferromagnesian mineral, biotite, hornblende and less abundant augite, in a groundmass frequently containing a feldspar, which is commonly highly altered, or, in some cases, a feldspathoid.
