Classification and chemical differentiation of igneous rocks

Igneous rocks are distinguished on contrasting chemical composition. Because the bulk-rock chemistry is reflected in the nature and proportions of the mineral present, we will utilize a simple classification based on the proportions of quarts, feldspars and dark ferromagnesian, minerals and on the composition of the plagioclase.

The most refractory minerals (i.e. those stable at the highest magmatic temperatures) form first and as the melt cools, crystallize, in some cases replacing earlier ones. This crystallization takes place discontinuously; formation of another group of minerals proceeds continuously, as in the feldspars.

Bulk compositions reflect differences in mineral proportions among the various rock types. The solid and the liquid can be separated at this stage as well. The process is known as igneous differentiation and explains the contrasts in compositions of melts and solids produces during either partial fusion and / or partial freezing.