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Dominant species
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Adularia
Adularia
Chemical
Formula
KAlSi3O8
Species
Silicates
Crystal
System
Monoclinic
Mohs
Scale
6
Specific
Gravity
2.55-2.63
Color
Colorless, Greenish, Grayish yellow, White, Pink
Streak
white
Luster
Vitreous, pearly on cleavage surfaces
Refractive
Index
nα = 1.518–1.520nβ = 1.522–1.524nγ = 1.522–1.525
Diaphaneity
Transparent to translucent
Cleavage
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.
Crystal Habit:Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance.
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), of plagioclase. 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 is found in low temperature hydrothermal deposits, in the Adula Alps of Switzerland. The largest documented single crystal of orthoclase was found in Ural mountains, Russia. It measured ~10×10×0.4 m3 and weighed ~100 tons.

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 luster 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 luster 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.

Orthoclase is one of the ten defining minerals of the Mohs scale of mineral hardness.


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