If a granite has a slowly cooling process, it has the potential to form larger crystals. There are also peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling. Anatectic melts are also produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. Nevertheless, the composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon – alkali feldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite. This process operates regardless of the origin of parental magmas to granites, and regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was. The final texture and composition of a granite are generally distinctive as to its parental rock. For instance, a granite that is derived from partial melting of metasedimentary rocks 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 studies of experimental petrology, which demonstrate that partial melting of mantle peridotite cannot produce granitic melts in any case. Although the fractional crystallisation of basaltic melts can yield small amounts of granites, such granites must occur together with large amounts of basaltic rocks. A-type granites were defined as to occur in anorogenic setting, have alkaline and anhydrous compositions. They show a peculiar mineralogy and geochemistry, with particularly high silicon and potassium at the expense of calcium and magnesium.