microcline
Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures than orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.
Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle is slightly less than right angles; hence the name "microcline" from the Greek "small slope." It is a fully ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase. Microcline is identical to orthoclase in many physical properties; it can be distinguished by x-ray or optical examination; viewed under a polarizing microscope, microcline exhibits a minute multiple twinning which forms a grating-like structure that is unmistakable.
Perthite is either microcline or orthoclase with thin lamellae of exsolved albite.
Amazon stone, or amazonite, is a beautiful green variety of microcline. It is not found anywhere in the Amazon basin, however. Spanish explorers who named it apparently confused it with another green mineral from that region.
A sodic alkali feldspar named anorthoclase also occurs; it is a crystalline solid solution of KAlSi3O8 and NaAlSi3O8, the sodium-aluminium silicate being in larger proportion.
See also
 List of minerals
References
Alkali feldspars U. Texas
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