"Igneous Rocks are classified is several ways, and methods of classification have evolved a lot over the past 100 years. Each classification is useful for a certain purpose and reflect a particular way of looking at igneous rocks.
All rock classifications are based on two criteria, however, mineral content of the rock, and texture (grain size). A complete classification must include both components, although with igneous rocks both components are usually built into one rock name, for example, granite is a light colored/coarse grained rock. (This is unlike sedimentary rocks where a name like "arkosic sandstone"specifies both components separately; arkose meaning lots of feldspar and sandstone being the texture.)
Here we examine three classifications beginning with the simplest and easiest to use (but also the most inaccurate) and proceeding to more useful, but more complex, classifictions:
A color/texture classification (below)
A Modal Classification: based on mineral composition/texture.
A Normative (Suite) Classification: based on the chemistry of the rocks."
See the entire article @ http://csmres.jmu.edu