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Orthoclase
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class="infobox" width=300>Orthoclase | | |
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Optical properties | Crystal System: | Monoclinic | | Color in PPL: | Colorless, commonly with a dusty or cloudy appearance; white to pink in hand-sample | | Pleochroism: | N/A | | Habit/Shape: | Can be anhedral or euhedral.
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| Orthoclase | |
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Optical properties | Crystal System: | Monoclinic | | Color in PPL: | Colorless, commonly with a dusty or cloudy appearance; white to pink in hand-sample | | Pleochroism: | N/A | | Habit/Shape: | Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. | | Relief: | Low negative relief | | Cleavage/Fracture Habit: | Has perfect cleavage on and good cleavage on . Cleavages intersect at 90°. It can be difficult to see cleavage in thin section due to orthoclase's low relief. | | Twinning: | Typically displays carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase. | | Birefringence: | Up to first order white (roughly 0.007) | | Extinction Habit/Angle: | Parallel extinction to cleavage | | Length Slow/Fast: | N/A | | Optic Sign: | Biaxial negative | | 2Vx: | 40-70° | | Alteration: | Commonly alters to sericite or clay. | | Distinguishing Characteristics: | Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx. | |
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. An alternate name is alkali feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.
Physical properties
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.
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.
Uses
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 jewelry. 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.
The gemstone commonly called rainbow moonstone is more properly a colorless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of color, although their value and durability do not greatly differ.
See also
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