Pegmatite Environment main content.

Pegmatite Environment

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Exhibition Text

Mineral materials (elements), once widely distributed throughout the original magma, sometimes become greatly concentrated in the residual liquid at a late stage in the cooling magma. During crystallization an unusually coarse-grained grouping of minerals results; many of these contain elements that are rare. Although pegmatites are small in volume when compared to other igneous rock types, they have yielded more than 400 different mineral species — about one-fifth of all the known minerals. Some of the minerals from these deposits support entire industries — feldspar in pottery and glass manufacture; muscovite mica in electrical equipment; columbite and tantalite in the manufacture of heat-resistant alloys.

Audio Transcript
Magmas are complex liquid bodies. And they often undergo an incredibly complex history of crystallization, or solidification. One product that sometimes forms is a solution that is very thin, watery, and exceptionally enriched in elements that are able to develop into a great variety of mineral species. These minerals, uniquely, are often large in size.

Such a fluid, such a magmatic fluid, is referred to as a pegmatitic fluid. And in the two cases a bit towards the right of this area are displayed minerals that are formed in this pegmatitic environment. One of the striking characteristics of these minerals is the exceptionally large sizes to which they may develop. And in one of the cases, we can see very large size crystal clusters of almandine garnets, or individual crystals of tourmaline, beryl, muscovite, and so on.

Pegmatitic fluids are not chemically simple but vary considerably in their makeup. One of the more interesting of these pegmatitic fluids is one that contains unusual concentrations of such rare elements as tantalum, niobium, cesium, thorium, yttrium, and, especially, lithium. The presence of the element lithium allows us to speak of these fluids as lithium pegmatite fluids. And when they crystalliz