K-feldspar
| Orthoclase | |
|---|---|
 |
|
| General | |
| Category | Silicate mineral |
|
Formula (repeating unit) |
KAlSi3O8 |
| Strunz classification | 9.FA.30 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/m |
| Identification | |
| Color | Colorless, greenish, greyish yellow, white, pink |
| Crystal habit | Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. |
| Twinning | Typically displays Carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase. |
| Cleavage | Has perfect cleavage on {001} and good cleavage on {010}. Cleavages intersect at 90°. It can be difficult to see cleavage in thin section due to orthoclase's low relief. |
| Mohs scale hardness | 6 (defining mineral) |
| Luster | Vitreous, pearly on cleavage surfaces |
| Streak | white |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.55–2.63 |
| Optical properties | Biaxial (-), 2V = 65–75 |
| Refractive index | nα = 1.518–1.520 nβ = 1.522–1.524 nγ = 1.522–1.525 |
| Birefringence | 0.0050–0.0060 |
| Dispersion | relatively strong |
| Extinction | parallel to cleavage |
| Diagnostic features | Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx. |
| Other characteristics | Low negative relief |
| References | |
Orthoclase, or orthoclase feldspar (endmember formula KAlSi3O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. It is a type of potassium feldspar, also known as K-feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.
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 KAlSi3O8 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 KAlSi3O8 is microcline.
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