Orthoclase, or orthoclase

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

(

endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...

formula K Al Si₃ O₈), is an important

tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

which forms

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

. The name is from the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

for "straight fracture", because its two cleavage planes are at right angles to each other. It is a type of alkali feldspar, also known as potassium feldspar or K-spar. The gem known as moonstone (see below) is largely composed of orthoclase.

Formation and subtypes

left, Orthoclase crystal twinning from the Organ Mountains in New Mexico">Organ_Mountains.html" ;"title="crystal twinning from the Organ Mountains">crystal twinning from the Organ Mountains in New Mexico Orthoclase is a common constituent of most granites and other felsic

s and often forms huge crystals and masses in

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...

. Typically, the pure potassium

of orthoclase forms a

solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

with

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicat ...

, the sodium endmember (NaAlSi₃O₈) of

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

. While slowly cooling within the earth, sodium-rich albite

lamellae Lamella (: lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology), a papery rib beneath a mushroom cap * Lamella (botany) * Lamella (surface anatomy), a plate-like structure in an animal * Lame ...

form by

exsolution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" ...

, 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 the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal sys ...

. Sanidine is common in rapidly cooled volcanic rocks such as

obsidian Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...

and felsic pyroclastic rocks, and is notably found in

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanite, aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava (or shallow intrus ...

s of the Drachenfels, Germany. The lower-temperature polymorph of KAlSi₃O₈ is

microcline Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during s ...

. Adularia is a low temperature form of either microcline or orthoclase originally reported from the low temperature

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

deposits in the

Adula Alps The Adula Alps, also known as the West Graubünden and Misox Alps, are a western Alpine mountain group, the part of the Lepontine Alps from the Lukmanier Pass, Lukmanier and St Gotthard Passes to the Splügen Pass. They lie mostly in Switzerlan ...

of Switzerland. It was first described by Ermenegildo Pini in 1781. The optical effect of adularescence in moonstone is typically due to adularia. The largest documented single crystal of orthoclase was found in the

Ural Mountains The Ural Mountains ( ),; , ; , or simply the Urals, are a mountain range in Eurasia that runs north–south mostly through Russia, from the coast of the Arctic Ocean to the river Ural (river), Ural and northwestern Kazakhstan.

in Russia. It measured around and weighed around .

Applications

Together with the other potassium feldspars, orthoclase is a common raw material for the manufacture of some

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

es and some

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

s such as

porcelain Porcelain (), also called china, is a ceramic material made by heating Industrial mineral, raw materials, generally including kaolinite, in a kiln to temperatures between . The greater strength and translucence of porcelain, relative to oth ...

, and as a constituent of scouring powder. Some intergrowths of orthoclase and

have an attractive pale luster 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 luster is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the

state gem Leaders of states in the U.S. which have significant mineral deposits often create a state mineral, rock, stone or gemstone to promote interest in their natural resources, history, tourism, etc. Not every state has an official state mineral, rock, ...

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

. 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 The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...

of mineral hardness, on which it is listed as having a hardness of 6.

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

's ''Curiosity'' rover discovery of high levels of orthoclase in Martian sandstones suggested that some

Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. Trends in the planet's portrayal have largely been influenced by advances in planetary science. It became the most popular celes ...

rocks may have experienced complex geological processing, such as repeated melting.

References

{{Authority control Potassium minerals Aluminium minerals Tectosilicates Monoclinic minerals Minerals in space group 12 Feldspar Gemstones

Formation and subtypes

Applications

See also

References