Orthoclase
A key feldspar mineral forming igneous rocks and gemstones.
Orthoclase, also called orthoclase feldspar (chemical formula KAlSi3O8), is a key tectosilicate mineral found in igneous rocks. Its name comes from the Ancient Greek for "straight fracture," referring to its two cleavage planes that meet at right angles. It belongs to the alkali feldspar group, often called potassium feldspar or K-spar, and is the main component of the gemstone moonstone.
This mineral is common in most granites and other felsic igneous rocks, and it frequently forms large crystals and masses in pegmatite. Pure orthoclase typically forms a solid solution with albite, the sodium endmember of plagioclase. During slow cooling underground, sodium-rich albite lamellae separate out through exsolution, leaving the remaining orthoclase richer in potassium. This intergrowth of the two feldspars is known as perthite. The high-temperature version of KAlSi3O8 is sanidine, found in quickly cooled volcanic rocks like obsidian and felsic pyroclastic rocks, notably in the trachytes of Drachenfels, Germany. The low-temperature version is microcline. Adularia is a low-temperature form of either microcline or orthoclase, first described in 1781 by Ermenegildo Pini from hydrothermal deposits in Switzerland's Adula Alps; the adularescence seen in moonstone is usually due to adularia. The largest known single orthoclase crystal, discovered in Russia's Ural Mountains, measured about 10 by 10 by 0.4 meters and weighed roughly 100 tonnes.
Orthoclase, along with other potassium feldspars, is used as a raw material for making some glasses and ceramics like porcelain, and as an ingredient in scouring powder. Certain intergrowths of orthoclase and albite have a pale luster and are called moonstone when used in jewelry. Most moonstones are translucent and white, though grey and peach varieties exist. In gemology, this luster is termed adularescence, often described as creamy or silvery white with a billowy quality. Moonstone is the state gem of Florida. The gem called rainbow moonstone is actually a colorless form of labradorite, distinguished from true moonstone by greater transparency and play of color, though their value and durability are similar. Orthoclase is one of the ten defining minerals on the Mohs hardness scale, rated at 6. NASA's Curiosity rover found high levels of orthoclase in Martian sandstones, suggesting that some Martian rocks may have undergone complex geological processes like repeated melting.
- formula
- KAlSi3O8
- hardness
- 6 (Mohs scale)
- cleavage
- Two planes at right angles
- type
- Tectosilicate, alkali feldspar
- notable_varieties
- Moonstone, adularia, sanidine, microcline
Lore & Background
Orthoclase, also known as orthoclase feldspar, is a tectosilicate mineral with the endmember formula KAlSi3O8, and it is a key component of igneous rocks. Its name derives from Ancient Greek, meaning "straight fracture," a reference to its two cleavage planes that intersect at right angles. This mineral belongs to the alkali feldspar group, often called potassium feldspar or K-spar. In appearance, orthoclase can form massive crystals, with the largest documented single crystal found in the Ural Mountains of Russia, measuring approximately 10 meters in size and weighing around 100 metric tons. It is a defining mineral on the Mohs scale of hardness, rated at 6. The gemstone moonstone is largely composed of orthoclase; this variety exhibits a pale, billowy luster known as adularescence, typically creamy or silvery white, though grey and peach shades occur. Moonstone is the state gem of Florida. Orthoclase is widespread in granites and other felsic igneous rocks, and it commonly forms large crystals in pegmatite. Its range extends to Martian geology, as NASA's Curiosity rover discovered high levels of orthoclase in Martian sandstones, suggesting complex geological processing like repeated melting. The mineral’s defining characteristic is its solid solution with albite (the sodium endmember NaAlSi3O8 of plagioclase). During slow cooling within the Earth, sodium-rich albite lamellae exsolve, enriching the remaining orthoclase with potassium and creating a perthitic intergrowth. Higher-temperature polymorphs include sanidine, found in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, notably in trachytes from Drachenfels, Germany. The lower-temperature polymorph is microcline. Adularia, a low-temperature form of either microcline or orthoclase, was first described in 1781 from hydrothermal deposits in the Adula Alps of Switzerland and is typically responsible for the adularescence in moonstone.
Reader's Guide
Orthoclase is significant both as a rock-forming mineral and as an industrial and gemological material. Together with other potassium feldspars, it is a common raw material for manufacturing some glasses, ceramics such as porcelain, and scouring powder. In gemology, intergrowths of orthoclase and albite with an attractive pale luster are called moonstone, characterized by a creamy or silvery white adularescence; it is the state gem of Florida. The mineral is also one of the ten defining minerals on the Mohs scale of hardness, rated at 6. Notably, NASA's Curiosity rover discovered high levels of orthoclase in Martian sandstones, suggesting that some Martian rocks may have experienced complex geological processing such as repeated melting.
Did You Know?
- The name orthoclase comes from Ancient Greek for 'straight fracture' because its two cleavage planes are at right angles.
- Orthoclase is one of the ten defining minerals on the Mohs scale of mineral hardness, with a hardness of 6.
- NASA's Curiosity rover discovered high levels of orthoclase in Martian sandstones, suggesting complex geological processing.
More in Minerals And Gems 1-23
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record