Orthoclase
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Orthoclase, or orthoclase feldspar (
endmember An endmember (also end-member or end member) in mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and ...
formula
K
K
Al
Al
Si
Si
3
O
O
8), is an important
tectosilicate Silicate minerals are rock-forming 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 na ...
mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

mineral
which forms
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock is ...
. The name is from the
Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). Ancient Greek was the language of an ...
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 A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

granite
s and other
felsic In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock is ...
s and often forms huge crystals and masses in
pegmatite A pegmatite is an igneous rock showing a very coarse Texture (geology), texure, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a si ...
. Typically, the pure potassium
endmember An endmember (also end-member or end member) in mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and ...
of orthoclase forms a
solid solution A solid solution describes a family of materials which have a range of compositions (e.g. AxB1−x) and a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to desc ...

solid solution
with
albite Albite is a . It is the sodium of the plagioclase series. It represents a plagioclase with less than 10% content. The pure albite endmember has the formula 38. It is a . Its color is usually pure white, hence its name from ''albus''. It is ...

albite
, the sodium endmember (NaAlSi3O8), of
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
. While slowly cooling within the earth, sodium-rich albite
lamellae Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology) File:Trompetenpfifferling Teufelskanzel.jpg, "False" gills of ''Craterellus tubaeformis'' A lamella, or gill, is a papery h ...
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 feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the ''plag ...
. The higher-temperature polymorph of KAlSi3O8 is
sanidine Sanidine is the high temperature form of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propert ...

sanidine
. 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. Obsidian is produced from felsic lava, rich in the lighter elements ...

obsidian
and felsic
pyroclastic 300px, USGS scientist examines Mount_St._Helens.html"_;"title="pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens">pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens_ File:Volcanic_Stone_3D.ogg.html" ;"titl ...
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 enriched with silica ...
s of the Drachenfels, Germany. The lower-temperature polymorph of KAlSi3O8 is
microcline Microcline (KAlSi3O8) is an important igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in ...

microcline
. 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 Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC ...
deposits in the Adula Alps of Switzerland. It was first described by
Ermenegildo Pini Ermenegildo Pini (1739–1825) was an Italians, Italian clergyman, naturalist, mathematician, geologist and philosopher. He belonged to the Barnabites, Barnabite Order and worked mainly in northern Italy. He attempted to examine scientific ideas on ...

Ermenegildo Pini
in 1781. The optical effect of
adularescence Adularescence is an optical phenomenon, similar to labradorescence and aventurescence, that is produced in gemstones such as Moonstone_(gemstone), moonstones. Description The effect of adularescence, also commonly referred to as ''Lustre_(mineralo ...

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 ˈgorɨ; ba, Урал тауҙары, ''Ural tauźarı'') or simply the Urals, are a mountain range that runs approximately from north to south ...
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, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspars and the ''alkali'' (potas ...
s, orthoclase is a common raw material for the manufacture of some
glass Glass is a non- crystalline, often transparency and translucency, 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 ...

glass
es and some
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide of rutile. Ti(IV) centers are grey; oxygen ce ...

ceramic
s such as
porcelain Porcelain () is a material made by heating substances, generally including materials such as , in a to temperatures between . The strength, and translucence of porcelain, relative to other types of , arises mainly from and formation of the ...

porcelain
, and as a constituent of scouring powder. Some intergrowths of orthoclase and
albite Albite is a . It is the sodium of the plagioclase series. It represents a plagioclase with less than 10% content. The pure albite endmember has the formula 38. It is a . Its color is usually pure white, hence its name from ''albus''. It is ...

albite
have an attractive pale
luster Lustre or Luster may refer to: Places * Luster, Norway, a municipality in Vestlandet, Norway ** Luster (village), a village in the municipality of Luster * Lustre, Montana, an unincorporated community in the United States Entertainment * Luste ...
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 Adularescence is an optical phenomenon, similar to labradorescence and aventurescence, that is produced in gemstones such as Moonstone_(gemstone), moonstones. Description The effect of adularescence, also commonly referred to as ''Lustre_(mineralo ...

adularescence
and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of
Florida Florida is a located in the region of the . Florida is bordered to the west by the , to the northwest by , to the north by , to the east by and , and to the south by the and ; it is the only state that borders both the Gulf of Mexico and ...

Florida
. The gemstone commonly called ''rainbow moonstone'' is more properly a colorless form of
labradorite Labradorite ((Ca, Na)(Al, Si)4O8) is a calcium-enriched feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the ' ...

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 Ordinal data is a categorical, statistical data type where the variables have natural, ordered categories and the distances between the categories are not known. These data ex ...
, 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, U.S. federal government responsible for the civilian Lis ...

NASA
's
Curiosity Rover ''Curiosity'' is a car-sized Mars rover designed to explore the Gale crater on Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In Englis ...
discovery of high levels of orthoclase in Martian sandstones suggested that some Martian rocks may have experienced complex geological processing, such as repeated melting.


See also

*
List of minerals This is a list of minerals for which there are articles on Wikipedia. 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 ...
*
Schiller, optical effect
Schiller, optical effect


References

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