Orthoclase, or orthoclase feldspar ( endmember formula K Al Si₃ O₈), is an important tectosilicate

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

which forms

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

. The name is from the

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

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.

Formation and subtypes

Orthoclase is a common constituent of most

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

s and other

felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, wh ...

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 silici ...

. Typically, the pure potassium endmember of orthoclase forms a solid solution 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 tectosilicate ...

, the sodium endmember (NaAlSi₃O₈), of

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...

. 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 Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, ...

. 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 s ...

. Sanidine is common in rapidly cooled volcanic rocks such as

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

and felsic

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

rocks, and is notably found in

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

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 durin ...

. 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 , topo_map= Swiss Federal Office of Topography swisstopo , photo=MonteLeone0001.jpg , photo_caption=Monte Leone , country_type= Countries , country= , subdivision1_type= Cantons, Regions , subdivision1= , parent= Western Alps , borders_o ...

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 ( ; rus, Ура́льские го́ры, r=Uralskiye gory, p=ʊˈralʲskʲɪjə ˈɡorɨ; ba, Урал тауҙары) or simply the Urals, are a mountain range that runs approximately from north to south through western ...

in Russia. It measured ~10×10×0.4 m and weighed ~100 tons.

Uses

Together with the other potassium

feldspar Feldspars are 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 ''plagioclase'' (sodium-calcium) felds ...

s, orthoclase is a common raw material for the manufacture of some

glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...

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, porcelain, ...

s such as

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...

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

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 Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and ...

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

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

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

Curiosity Rover ''Curiosity'' is a car-sized Mars rover designed to explore the Gale crater on Mars as part of NASA's Mars Science Laboratory (MSL) mission. ''Curiosity'' was launched from Cape Canaveral (CCAFS) on November 26, 2011, at 15:02:00 UTC and la ...

discovery of high levels of orthoclase in Martian sandstones suggested that some Martian 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

Uses

See also

References