teaching hand/display specimen of a quartz/muscovite/microcline pegmatite from the Tin Mountain 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 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.
This specimen was collected near the long-inactive Tin Mountain Mine near Custer, South Dakota. It was a large producer of feldspar for use in ceramics in the 1930s and was initially mined for the lithium-bearing mineral spodumene, with intermittent mining of this mineral into the 1970s. The pegmatite intrudes the early Proterozoic schist of the Mayo Formation and is thought to be 1.7 billion years old.
This specimen is primarily quartz and muscovite, with minor pale pink microcline. Excellent specimen to show the texture of a pegmatite.
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