Orthoclase

Orthoclase (endmember formula KAlSi₃O₈) is an important tectosilicate mineral, which forms igneous rock. It is also known as alkali feldspar and is common in granite and related rocks.

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Orthoclase is named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly luster. It can be colored white, gray, yellow, pink, or red; rarely green. Twinned crystals are quite common. Orthoclase is a common constituent of most granites and other felsic igneous rocks and is often found in huge crystals and masses in pegmatite masses.

Typical orthoclase is a solid solution between the pure potassium endmember and the sodium endmember, albite (NaAlSi₃O₈). During slow cooling within the earth, sodium-rich albite lamellae form by exsolution and the remaining orthoclase becomes more potassium-rich. 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. A notable locality with sanidine is in the trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline.

Adularia (from Adular) is found in low temperature hydrothermal deposits. When pearly and opalescent, orthoclase is called moonstone and is used in jewelry. These opalescent varieties are known to be an intergrowth of orthoclase and albite.

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.

It is the state gem of Florida.  

See also

• List of minerals

References

• Alkali feldspars U. Texas
• Mindat