feldspar - display specimen of bright salmon pink microcline from Custer County, South Dakota
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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.
Generally, if a potassium feldspar is colored, it is microcline. Iron causes a pink coloration. If microcline is blue-green, the variety is called amazonite, though it was never found in the Amazon. Amazonite's color is caused by minute quantities of lead and water acted on by natural gamma radiation, primarily from 40K.
This microcline was collected over 30 years ago from a pegmatite mined by Wyoming Red Rock north of Pringle, South Dakota. It is strongly perthitic, with irregular lamellae of sodium feldspar in a potassic alkali feldspar that represents unmixing (exsolution) during crystallization. The pencil is 5" long, for scale. There are two photos the same specimen.
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