Most geologists would define pegmatites as exceedingly coarse-grained igneous rocks of granitic composition. They tend to be enriched in normally incompatible trace elements, which gives rise to a large number and variety of uncommon minerals. This definition would probably encompass about 80% of the rock that is called pegmatite on geologic maps and in petrologic studies. Perhaps that’s as good as any definition can hope to be, but which of these features actually defines pegmatite?

Giant grain size in relation to common igneous and metamorphic rocks – the hallmark of pegmatites – is not necessary. Pegmatites can actually be very fine grained. Micropegmatites and granophyres (Fig. 1) are recognized as pegmatitic, though their grain size ranges from the millimeter to centimeter scale. Depending on your point of view, igneous is not necessary, either. Geologists have proposed hydrothermal or metamorphic origins for pegmatites since the 19th century, and this concept has maintained its few but very credible advocates over the years. Though most pegmatites are granitic, this is not a necessary restriction. Pegmatites of the composition of basic and alkalic igneous rocks are common, though in relation to the gabbros and syenites that host them, they are far subordinate to granitic pegmatites.

One conclusion to draw is that pegmatites are not, by definition, any particular rock. They are a combination of features including grain size, fabric (Fig. 2), and composition that, collectively, lead petrologists to distinguish them in some way from ordinary igneous rocks and from hydrothermal veins that are associated with most metallic ore deposits.