A variety of classes of granites have been established by geologists, most of which are differentiated by the relative coarseness of the grain or the amounts of the constituent minerals that are present. Granites are, however, chiefly composed of plagioclase and alkali feldspars. Other minor minerals that are commonly incorporated in the rocks include muscovite, pyroxenes, and biotite. Indeed, biotite is almost always found in granites of all types, though sometimes in negligible quantities.