Systems of nomenclature and classification may reflect:
genetic, textural, chemical or mineralogical features.
plutonic - at depth
hypabyssal - intermediate depth
volcanic - on the Earth's surface.
This system is not very practical, but it serves as a first approximation, it tells nothing about mineralogy, chemistry of the rocks and can not distinguish basalt from rhyolite.
aphanitic - fine grained < 1 mm
phaneritic - medium grained 1 to 5 mm
coarse grained (pegmatitic) > 5 mm
This system has the same shortcomings as a genetic classification, however specific textures present may aid in classification, e.g., phenocryst, ophitic, coronas, but these are not indicative of a specific environment of formation or a specific lithology.
This type of classification requires a complete chemical analysis of the rock in order to pigeonhole a sample, and is not practical under field conditions where only a hand lens and hammer are available. A chemical classification system has been proposed for volcanic rocks and a comparable scheme for plutonic rocks is not available.
This leaves us with a system based on mineralogy.
This is based on Modal Mineralogy (MODE - an accurate representation of the distribution and volume percent of minerals within a given rock).
The system is simple to use, can be applied in the field, using a hand lens and a hammer. It is based on the percentages of Q (quartz), A (alkali feldspar), P (plagioclase), and F (feldspathoids, e.g. nepheline, leucite). Further subdivisions are dependant on the type and percentage of mafic minerals present.
Two systems have been proposed - Handouts.