| GEMS & MINERALS FROM PEGMATITES I. What is a Pegmatite? David London School of Geology & Geophysics University of Oklahoma 100 East Boyd Street, Rm 810 SEC Norman, OK 73019 Email: [email protected] Many of the world's finest gem and mineral specimens, especially those with impressive color, size, and form, are mined from granitic pegmatites. Pegmatites are the primary sources for these familiar minerals and their colored varieties: � aquamarine, emerald, heliodor, and morganite, all varieties of the mineral beryl � rubellite, verdelite, indicolite, and watermelon, all varieties of tourmaline � spessartine and almandine species of garnet � precious (blue) topaz � crystal, rose, and smoky varieties of quartz � amazonite, the blue-green variety of microcline feldspar � kunzite and hiddenite, the gem pink and green varieties of spodumene. And the list goes on to many more exotic but nonetheless beautiful minerals. Here and in subsequent issues of the Sooner Rockologist, I will offer short articles about pegmatites, their minerals, and their origins. This topic is the focus of my research, and it is a field in which many collectors and amateur scientists can and do participate. I am the world representative of the Pegmatite Interest Group (PIG), and I manage a web site, hosted by the Mineralogical Society of America, at: � http://www.minsocam.org/msa/special/Pig/ where anyone interested in the minerals and gems from pegmatites can go to learn, comment, or just enjoy the postings. I list news, events, photos, and short articles on pegmatites, including recent mining efforts that have resulted in major discoveries of tourmaline in California, Madagascar, and elsewhere. Our logo, "Get PIG-headed", is an invitation to all of you to get hooked on these great rocks. What is a Pegmatite? The term pegmatite, which originates from a Greek word that refers to an "intimate mixture", was first applied in the early 19th century to the intergrowth of quartz within microcline feldspar known as "graphic" or "cuneform" granite (Figure 1). Though pegmatites of different composition exist, most are essentially granites. This means that pegmatites contain nearly equal proportions of quartz, albite (soda) feldspar, and microcline (potash) feldspar. They are distinguished from granites by their coarse grain size, with giant crystals measurable in meters in some very large pegmatites. One other feature of texture distinguishes pegmatites from granites - whereas granites are very uniform in their composition and texture (which makes them good materials for building stone), pegmatites are highly variable. Granitic pegmatites are thought to represent the last portions of magma - molten silicate material - present in large masses of granite. If that last fraction of melt remains within its source granite, it may form small pegmatite masses near the top of the granite body. More commonly, however, that last bit of melt is forced out of the granite crystal mush along cracks in the surrounding rocks, in which it eventually freezes to form pegmatite dikes. Pegmatite dikes of ordinary granitic composition are found closest to their sources, but the farther they migrate from source, the more exotic they become in composition, and hence in mineralogy. Usually, the pegmatites that contain gem minerals are associated with shallow levels of emplacement and crystallization, corresponding to depths of about 2-6 kilometers below surface. (Next: Formation of Gem-bearing Pockets) |