Pegmatites Contain Very Large Crystals

Pegmatites are extremely coarse-grained igneous rocks, usually of granitic or granodioritic composition (Figure). Granitic and granodioritic pegmatites typically are dominated by quartz, alkali feldspars, biotite, and muscovite. They probably form dur-ing the top stages of crystallization, when remaining magma is enriched in volatiles and elements that don't easily enter into common rock-forming minerals. Such elements include boron,cesium, beryllium, zirconium, niobium, uranium, thorium, tan-talum, tin, rare-earth element elements, chlorine, fluorine, lithium, and phosphorus. The presence of abundant fluids and uncommonelements often leads to the formation of huge , unusual, and sometimes valuable minerals. Gemstones from pegmatites include tourmaline, bright green feldspar (amazonite), beryl, chrysoberyl, topaz, cryolite, and spodumene.

The largest crystals within the world are found in peg-matites. one crystal of phlogopite from Ontario, Canada, was described as being 4.2 m (14 ft.) wide and 10 m (33 ft.) long. A quartz from a Russian pegmatite weighed quite 907 kg (2,000 lbs.). the most important quartz on record, however, was from Brazil and weighed quite five tons. Pegmatites within the Black Hills of South Dakota (Figure) are famous for “logs” of white spodumene, which are up to fifteen .2 m (50 ft.) long. In some Black Hills pegmatites, mineralogists have found tourmaline crystals that are 2.7 m (9 ft.) long, topaz crystals that weigh many pounds, and amblygonite crystals that are several feet across.

Gem-grade tourmalines in pegmatites are available a spread of colours . the foremost common sort of tourmaline is that the black or iron-rich variety, called schorl. Brown tourmaline (dravite) contains magnesium. Lithium is primarily liable for the green (verdelite), yellow, pink (rubellite), and blue (indicolite) varieties. Colorless tourmaline, called achroite, is rare. Individual tourmaline crystals can also contain zones of various colors, which resulted from chemical changes during the expansion of the crystal (Plate 1.1). the foremost famous multicolor tourmalines are the “watermelon” variety, which contains a red core with green “rinds.”

Most spodumene in pegmatites is white and opaque. However, the rare gem variety, kunzite, is pink and transparent. Emerald-green spodumene is named hiddenite and was named after W. E. Hidden, who first discovered the mineral when he overturned a tree on a farm near Stony Point, North Carolina. Beryl from pegmatites is typically yellow-green to blue-green. One sort of beryl is aquamarine, which varies from transparent blue to green . Heliodsor refers to the golden-yellow sort of beryl that's found in Namibia. Chrysoberyl, whose color ranges from white to yellowish-green to green, is found during a number of various lithologies, but a number of the simplest specimens are found in pegmatites. One sort of chrysoberyl, alexandrite, has the weird property of being emerald green in daylight and red in artificial light.

Figure 

The wall of the Tin Mountain Pegmatite Quarry, Black Hills, South Dakota. The geologist is standing ahead of an outsized , thin crystal of spodumene that's several meters long. Other large crystals, mostly spodumene, also can be seen during this photograph. Many pegmatites contain crystals much larger than shown here.