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 crystallize, a lithium pegmatite develops.

Such a fluid is chemically very active and usually interacts with those minerals that had already formed. Look at specimen number 12, an impressive large crystal of beryl. Notice how it's been etched by the remaining fluids, rich in lithium, that had begun to dissolve some of the already-formed beryl.

In this way, you see, minerals that had already formed are able to go back into solution. And through this complex interplay of chemical reaction, there develops a variety of minerals to produce a rock body we speak of as a lithium replacement pegmatite.

They are a complex pegmatite, in the sense that the minerals, such as the beryl, lepidolite, and spodumene, reveal a complex history of development. In other complex pegmatites lithium, yttrium, and cesium are not the primary elemental constituents involved. Instead, uranium and phosphorus dominate in the pegmatitic fluid.

And so, in the next case to the right of the lithium replacement pegmatite can be seen a collection of minerals that are typically formed in a complex pegmatite, one that is rich in the elements uranium and phosphorus. Notice the presence of such minerals as urnainite and apatite. In the next two cases to your right are to be seen some rather impressive and especially large-sized crystals of elbaite tourmaline, spodumene topaz, microcline feldspar, and almandine garnet.

These minerals clearly present you with the idea that the pegmatite rock body, as unusual as it is in terms of its chemical composition, is perhaps most strikingly appreciated by the large-size mineral assemblages that have the opportunity to form. Just why minerals develop into sizes as large as the ones you see here, or even larger, for that matter, is a rather difficult question to answer.

In all probability, the pegmatitic fluids cool very, very slowly and thereby provide for the large crystalline structures necessary to develop in these minerals. Possibly too, the atoms have great freedom of mobility within the thin pegmatitic fluid, allowing for large crystalline structures to form.

Specimen number 10, an almandine garnet, interestingly, is from New York City, from the area around 66th street and Broadway, where many pegmatites can be found.