Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.
Formation and subtypes
Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.
Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8). While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.
The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia (from Adular) is found in low temperature hydrothermal deposits. The largest documented single crystal of othoclase was found in Ural mountains, Russia. It measured ~10x10x0.4 m3 and weighed ~100 tons.
Together with the other potassium feldspars orthoclase is a common raw material for the manufacture of some glasses, some ceramics, such as porcelain, and as a constituent of scouring powder.
Some intergrowths of orthoclase and albite have an attractive pale lustre and are called moonstone when used in jewellery. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. In gemology, their lustre is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of Florida.
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