Granite may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based. The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources). Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks. M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. However, this proposal has been rejected by
Alkali feldspar granite Less than that amount would designate the rock as a granite. Other incorporated silicate minerals may include, very minor amounts of plagioclase feldspar, mica in the form of muscovite and/or biotite, and amphibole (often hornblende). Oxide minerals such as magnetite, ilmenite, or ulvospinel. Some sulfides and phosphates (mainly apatite) may also be present. Alkali feldspar granites generally occur with other alkali-rich granitoids, such as monzogranite and syenogranite, forming part of the A-type granites. They are found in a wide range of tectonic settings and their origins remain uncertain. Granitic rocks in general are of interest to geologists, geochemists, etc., because