feldspar - teaching student specimens of pale pink perthitic microcline from the Mtn Beryl Mine - Unit of 5 specimens
microcline K(AlSi3O8) Custer County, South Dakota
Microcline and orthoclase are compositionally the same and can only be separated with the use of a microscope or with x-ray diffraction. Occurrence helps - generally if found in large crystals in a pegmatite, the feldspar is microcline. Orthoclase most commonly occurs as phenocrysts in igneous rocks, with the difference lying in the rate of crystallization, since microcline crystallizes more slowly at lower temperatures than orthoclase and is often colored. These two, along with sanidine and anorthoclase, form the potassium feldspar group.
Under a polarizing microscope, microcline often shows striations on cleavage surfaces due to lamellar twinning. This does not occur in orthoclase. There are two good directions of cleavage, meeting at an 89˚ angle, and one poor direction of cleavage, where the feldspar breaks between rows of atoms. These attractive specimens all clearly show the two good cleavage faces. The name microcline is derived from the Greek words for little and inclined, referring to the slight departure in cleavage direction from 90˚.
Many pegmatites in the Black Hills are composed primarily of quartz, microcline and muscovite. The feldspar, when present in commercial quantities, is mined for use in ceramics.
Chemically, sodium can replace potassium to form soda-microcline. When there is more sodium than potassium, the mineral is called anorthoclase.
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.
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