pegmatite - large display specimen of quartz/muscovite/microcline pegmatite from the Mtn. Beryl Mine, Black Hills, South Dakota

Pegmatites are essentially giant crystal granites, formed from mineral-rich fluids that make up the last stages in the emplacement of an igneous body. Pegmatites have crystals greater than a centimeter in diameter, easily visible at greater than arm’s length. These are typically quartz, microcline feldspar, and micas, with beryl, tourmaline, spodumene in lesser amounts. As a magma crystallizes and crystals of various minerals fall to the floor of the magma chamber or float around in the magma, the residual magma becomes more and more water rich and concentrated in the ions that do not make up the composition of the common rock-forming minerals. Why water-rich? Water is a  component of most magmas, only crystallizing when temperatures fall below 32˚F, at which point it becomes a mineral.

These water-rich mineralized fluids, sometimes called mother liquors, are injected into crevasses and cavities in the rock overlying an intrusion. Since the remaining ions have greater mobility, they rapidly form much larger crystals than those of the granitic rock that is crystallizing at depth. Crystals of spodumene 40 feet long have been mined from the pegmatites in the Black Hills, of South Dakota. 

Spodumene is mined for lithium and beryl for beryllium, and are practically the only sources for these metals. Some pegmatites are mined entirely for feldspar or mica, while others are mined for gemstones such as apatite, tourmaline and topaz.

By 1950, the Mountain Beryl Mine, about two miles northwest of Pringle, South Dakota, had produced roughly three tons of beryl for the metal beryllium and 10 tons of specimen beryl, 2 1/2 tons of scrap mica and an unrecorded amount of feldspar from two pegmatite dikes. Beryl crystals as large as 18 inches by 4 feet were reported, with beryl crystals 6 inches in diameter fairly abundant. The mine has long been inactive, with large trees growing on the tailings and the main pit flooded.

This pegmatite is primarily quartz, muscovite and minor microcline.  A textbook example of a pegmatite.  Both sides of the specimen with the black pencil in front are shown. The pencil is 5 1/2" long for scale. We don't have large specimens like these very often, as many of the mines in the Black Hills are inactive.

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