Students of EPS 2: Examinations will not include this information:
The most spectacular pegmatites contain abnormally large crystals mixed with medium sized and smaller crystals. Crystals up to many meters long have been reported.
Some pegmatite bodies are small irregular patches less than 1 cm. (0.4 in) across in larger masses of plutonic or metamorphic rocks. Others may be thousands of meters in length and hundreds of meters thick. Some appear as dikes, veins, or sills. Many have irregular outlines.
Most pegmatites show symmetrical internal zonation.
Pegmatites may be composed of a variety of minerals. Terms such as granite pegmatite, gabbro pegmatite, syenite pegmatite, or names with any other plutonic rock type as prefix are used.
Compositions in the range from granodiorite to granite are common. Large crystals of quartz, potassium feldspar, sodium rich plagioclase, and micas (e.g., muscovite and lepidolite) may be abundant.
Simple pegmatites contain few, if any, exotic minerals. The center zones of complex pegmatites, however, may contain a wide variety of minerals such as tourmaline, topaz, garnet, spodumene, scapolite, beryl, apatite, fluorite, zircon, and various rare minerals some limited to only a few localities in the world. GEM quality stones are sought in such rocks.
Elements such as tungsten, boron, tantalum, columbium, bismuth, tin, uranium, radium, sheet mica, and sulfide minerals of various metallic elements are among substances obtained from pegmatite deposits.
Pegmatites have provided radioactive minerals for use in radiometric age dating of many rock complexes.
As a result of the bewildering variety of shapes, sizes, appearances, and field relationships, many origins have been proposed for pegmatites. Some dikelike bodies showing clear INTRUSIVE relationships must be of igneous origin. These frequently cut across all other associated rocks and therefore represent material from late stages of crystallization of plutonic complexes. They were probably rich in volatile materials such as water, fluorine, chlorine, phosphorus, and sulfur. This highly fluid, aqueous melt provided an environment for concentration of chemical elements with ionic sizes too great to fit into crystalline structures of major rock forming minerals; these elements were thus concentrated in pegmatite deposits.
The occurrence of pegmatite corresponding to most plutonic rock compositions gabbros, diorites, syenites, anorthosites further recommends this possibility.
Other pegmatites grade into the rocks that surround them and show no intrusive relationships. Such bodies may represent material produced by melting (anatexis) during metamorphism at high temperatures and pressures. Some elements and fluids may be literally "sweated out" of a rock complex during metamorphism. well known because it contains crystals of many different minerals. This rock is pushed up as large veins of magma that was rich in volatile elements resulting in large crystals, usually surrounded by grantic rocks.