pegmatite - teaching hand specimen of a quartz/muscovite/microcline pegmatite from the Mtn. Beryl Mine, Black Hills, South Dakota

$ 7.50

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.

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.

These specimens are primarily quartz, muscovite and pale pink microcline. The black crystals are schorl, the black variety of tourmaline. Excellent small teaching hand specimens to show the texture of a pegmatite.

 

 

Select a specimen: You can select a specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.

Shipping: in a First Class package unless combined with other items. Click > Shipping < for shipping rates. Use back button to return to this page.