feldspar - Display specimen of pale pink perthitic microcline from the Mountain Beryl Mine, Custer County, SD
Microcline K(AlSi3O8) Mountain Beryl Mine, Custer County, South Dakota
This pale pink feldspar is microcline. It is frequently misidentified as orthoclase.
Microcline and orthoclase have the same hardness, are compositionally the same and can only be separated with the use of a microscope or x-ray diffraction, but if found in large crystals in a pegmatite, the feldspar is microcline. If found as phenocrysts in an igneous rock, the feldspar is orthoclase, with the difference lying in the rate of crystallization, since microcline crystallizes at lower temperatures than orthoclase and is often colored. These two, along with sanidine and anorthoclase, form the Potassium Feldspar group. Chemically, sodium can replace potassium to form soda-microcline. When there is more sodium than potassium, the mineral is called anorthoclase.
Under a polarizing microscope, microcline often shows striations on cleavage surfaces due to lamellar twinning. This does not occur in orthoclase. This microcline shows a perthitic intergrowth of albite as white lamellae and veins, common in Black Hills microcline.
Microcline has 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 excellent display specimens clearly shows these cleavage directions.
The name microcline is derived from the Greek words for little and inclined, referring to the slight departure in cleavage from 90˚. Perthite is named for the city of Perth, Ontario. It was originally described from North Burgess, Drummond Township, Lanark Co., Ontario, Canada.
Collected from the Mountain Beryl Mine, about two miles northwest of Pringle, South Dakota, which, by 1950 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.
The pencil is 5 inches long for scale. These hefty cleavage specimens will stand on edge for display.
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