pegmatite - display specimen of quartz/muscovite/microcline pegmatite from the Tin Mountain Mine, Custer County, 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 mineral also and a component of most magmas, only crystallizing when temperatures fall below 32˚ F, an unlikely occurrence underground.
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.
The Tin Mountain Mine claim was patented in 1889. Not much tin was ever produced, but huge quantities of spodumene were mined for its lithium content. Tonnages of microcline were mined for use in ceramics. The mine is no longer active, though it has recently been purchased with an eye on possibly reworking the tailings and is now closed to collecting.
This pegmatite is primarily quartz, muscovite and with some pale pink microcline. Textbook pegmatite in a display specimen, with thick muscovite books, pink microcline and glassy quartz.
The pencil is 5" long, for scale
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