This topaz illustrates the large-size crystals typical of the pegmatites. Material of gem quality is characterized by the blue-green topaz.
Development of unusually large crystals is a special feature of minerals forming in a pegmatite environment. Individual crystals may weigh as much as 50 tons and measure more than 40 feet in length. Tourmaline, beryl, and topaz illustrate the large-size crystals typical of the pegmatites.
Why such unusually large-sized mineral crystals can develop in a pegmatite environment has not been fully explained. Pegmatite fluids are not as thick and viscous as the original magma from which they were derived, so that atoms of the dissolved elements can move about freely. In this way, crystalline structures which build to immense proportions are thought to develop. Pegmatitic fluids also undergo an unusually slow rate of cooling that allows sufficient time for large-sized crystalline structures to form. Because pegmatite minerals are so large in size, many exceedingly rare, many of gem quality and others of excellent crystal form, the numerous varieties of pegmatite minerals are perhaps the most exciting to observe and study.
Minas Gerais, Brazil