feldspar - teaching student specimens of pale pink to tan perthitic microcline - 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.
Microcline often shows microscopic 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 tan student specimens all clearly show the two good cleavage faces. Locally called "perthite," many faintly exhibit thin lamellae of albite, which exsolves from microcline when cooled below 650˚C, the temperature of mutual solubility. Perthite is common in granitic pegmatites.
The name microcline is derived from the Greek words for little and inclined, referring to the slight departure 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.
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 The mine is no longer active.
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