muscovite mica from the Mtn. Beryl Mine, Black Hills, SD - hand specimen

muscovite mica      KAL₃Si₃O₁₀(OH)₂

Muscovite is a common rock-forming mineral. In pegmatites, which are essentially giant-crystal granitic rocks, muscovite forms large books and can be cleaved into large sheets.  It is mined commercially for use as an insulator in transformers and electrical equipment. Scrap muscovite, small flakes, is used as a filler or extender in paints. Transparent in thin sheets, muscovite is flexible and also elastic - it springs back when released.

The large crystals of pegmatites form in the final stages of the crystallizing of a magma. As crystallization progresses and various minerals are removed from the melt, the magma becomes more and more water-rich. This allows atoms to move around rapidly and causes the formation of large crystals. These “mother liquors” are often injected into fissures above the magma chamber to form pegmatite dikes.

In the Black Hiils of South Dakota, famous for its pegmatites, crystals of spodumene 40 feet long occur. Huge masses of microcline feldspar are common. This muscovite was collected from a pegmatite at the Mountain Beryl Mine in Custer County, South Dakota. The mine is no longer active.

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