feldspar - microcline - bright salmon pink perthitic microcline from Custer County, South Dakota - hand specimen
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.
The perthitic texture is caused by an intergrowth of two feldspars. The white intergrown lamellae are albite.
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 from a pegmatite roughly one mile north of Pringle, South Dakota.
Select a specimen: If more than one specimen is shown, you can select a particular 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.
Shipping: in a First Class package unless combined with other items. Click > here < for shipping rates. Use back button to return to this page.
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.