Pegmatites are holocrystalline rocks typically composed of igneous rock-forming minerals that are, in part, very coarse grained, although some are extremely varied in grain size (Jahns, 1955). They are commonly granitic in composition, consisting mainly of quartz, feldspar, and mica, but more mafic varieties composed of olivine, pyroxene, and plagio- clase also occur. Mafic pegmatites, however, have negligible economic significance and are not considered in this review.

A general classification scheme for granitic pegmatites based on their environments of formation and mineralogi- cal features as suggested by Ginsburg (1984) and modified by Cerny (1990, 1991b), is summarized in Table 21-1. Abyssal and muscovite class pegmatites are commonly mineralogically simple. Abyssal pegmatites consist mainly of quartz and feldspar, and are generally barren or poorly mineralized with regard to rare elements such as niobium, tantalum, rare-earth elements, yttrium, and beryllium. Muscovite class pegmatites contain extensive reserves of mica and feldspar and are in some cases enriched in uranium and rare-earth elements. Many of the pegmatites of the northeastern and central Grenville zones, Ontario likely belong to the muscovite class, but others contain significant rare element minerals and for these a transitional classification between muscovite class and rare element pegmatites is probably more appropriate (Cerny, 1990). Miarolitic (gem-bearing) pegmatites are extremely rare in Canada.