Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspars and the ''alkali'' (potassium-sodium) feldspars. Feldspars make up about 60% of the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

and 41% of the Earth's

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

by weight. Feldspars

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

lize from

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

as both intrusive and

extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contr ...

igneous 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 ...

rocks and are also present in many types of

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

. Rock formed almost entirely of calcic

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 ...

feldspar is known as

anorthosite Anorthosite () is a phaneritic, intrusive rock, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic ...

. Feldspars are also found in many types of

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

Etymology

The name ''feldspar'' derives from the German , a compound of the words ' ("field") and ("flake"). had long been used as the word for "a rock easily cleaved into flakes"; was introduced in the 18th century as a more specific term, referring perhaps to its common occurrence in rocks found in fields (Urban Brückmann, 1783) or to its occurrence as "fields" within granite and other minerals (René-Just Haüy, 1804). The change from to ''-spar'' was influenced by the English word ''spar'', meaning a non-opaque mineral with good cleavage. ''Feldspathic'' refers to materials that contain feldspar. The alternate spelling, ''felspar'', has fallen out of use. The term "felsic", meaning light coloured minerals such as quartz and feldspars, is an acronymic word derived from ''fel''dspar and ''si''lica, unrelated to the obsolete spelling "felspar".

Compositions

The feldspar group of minerals consists of

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 ...

s, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: *

orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), 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 ...

endmember K Al Si OFeldspar
What is Feldspar?
Industrial Minerals Association. Retrieved on July 18, 2007. *

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 ...

endmember NaAlSiO *

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

CaAlSiO

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 ...

s between orthoclase and albite are called alkali feldspar. Solid solutions between albite and anorthite are called

, or, more properly, plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both a plagioclase and an alkali feldspar. The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, is the basis for the

QAPF classification A QAPF diagram is a doubled-triangle plot diagram used to classify igneous rocks based on their mineralogy. The acronym QAPF stands for "quartz, alkali feldspar, plagioclase, feldspathoid (foid)", which are the four mineral groups used for cla ...

of igneous rock. Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, then the plagioclase becomes increasingly sodium-rich as crystallization continues. This defines the continuous Bowen's reaction series. K-feldspar is the final feldspar to crystallize from the

Alkali feldspars

Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these includes: *

(

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

)"The Mineral Orthoclase"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. *

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 Feldspar"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. *

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 ...

( triclinic)"Microcline Feldspar"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. * anorthoclase ( triclinic) Potassium and sodium feldspars are not perfectly

miscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). Such substances are said to be miscible (etymologically ...

in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments. Sanidine is stable at the highest temperatures, and microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to

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" ...

of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, 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 coo ...

s can be seen with the naked eye. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Ammonium feldspar

Buddingtonite is an ammonium feldspar with the chemical formula: NH₄AlSi₃O₈. It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.

Barium feldspars

Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Barium feldspars are sometimes classified as a separate group of feldspars, and sometimes they are classified as a sub-group of alkali feldspars. The barium feldspars are monoclinic and include the following: *

celsian Celsian is an uncommon feldspar mineral, barium aluminosilicate, Barium, BaAluminium, Al2Silicon, Si2Oxygen, O8. The mineral occurs in contact metamorphic rocks with significant barium content. Its crystal system is monoclinic, and it is white, ...

hyalophane Hyalophane or jaloallofane is a crystalline mineral, part of the feldspar group of tectosilicates. It is considered a barium-rich potassium feldspar. Its chemical formula is , and it has a hardness of 6 to . The name ''hyalophane'' comes from the ...

Plagioclase feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

(0 to 10) *

oligoclase Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite:ano ...

(10 to 30) * andesine (30 to 50) * labradorite (50 to 70) * bytownite (70 to 90) *

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

labradorite composition is due to very fine-grained exsolution

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 ...

known as Bøggild intergrowth. The

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

in the plagioclase series increases from

(2.62) to anorthite (2.72–2.75).

Structure

The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Each oxygen ion, in turn, is shared by a neighbouring tetrahedron to form a three-dimensional network. The structure can be visualized as long chains of aluminosilicate tetrahedra, sometimes described as

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

chains because their shape is kinked. Each crankshaft chain links to neighbouring crankshaft chains to form a three-dimensional network of fused four-member rings. The structure is open enough for cations, typically sodium, potassium, or calcium, to fit into it and provide charge balance. File:Anorthite crankshaft.png, Diagram showing part of a crankshaft chain of feldspar File:Anorthite c.png, Feldspar crystal structure viewed along the c axis File:Anorthite a.png, Feldspar crystal structure viewed along the a axis File:Anorthite b.png, Feldspar crystal structure viewed along the b axis

Weathering

Chemical weathering of feldspars happens by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

and produces

clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minera ...

s, including

illite Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandw ...

smectite A smectite (; ; ) is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secon ...

, and

kaolinite Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

. Hydrolysis of feldspars begins with the feldspar dissolving in water, which happens best in acidic or basic solutions and less well in neutral ones. The speed at which feldspars are weathered is controlled by how quickly they are dissolved. Dissolved feldspar reacts with H⁺ or OH⁻ ions and precipitates clays. The reaction also produces new ions in solution, with the variety of ions controlled by the type of feldspar reacting. The abundance of feldspars in the Earth's crust means that clays are very abundant weathering products. About 40% of

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 ...

s in

s are clays and clays are the dominant minerals in the most common sedimentary rocks,

mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

s. They are also an important component of

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

s. Feldspar that has been replaced by clay looks chalky compared to more crystalline and glassy unweathered feldspar grains. Feldspars, especially plagioclase feldspars, are not very stable at the Earth's surface due to their high formation temperature. This lack of stability is why feldspars are easily weathered to clays. Because of this tendency to weather easily, feldspars are usually not prevalent in sedimentary rocks. Sedimentary rocks that contain large amounts of feldspar indicate that the sediment did not undergo much chemical weathering before being buried. This means it was probably transported a short distance in cold and/or dry conditions that did not promote weathering, and that it was quickly buried by other sediment. Sandstones with large amounts of feldspar are called arkoses.

Applications

Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paints, plastics, and rubber. In the US, about 66 % of feldspar is consumed in glassmaking, including glass containers and glass fibre. Ceramics (including electrical insulators, sanitaryware, tableware and tile) and other uses, such as fillers, accounted for the remainder.Apodaca, Lori E. (2008)
Feldspar and nepheline syenite
USGS 2008 Minerals Yearbook

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 ...

: Feldspar provides both K₂O and Na₂O for fluxing, and Al₂O₃ and CaO as stabilizers. As an important source of Al₂O₃ for glassmaking, feldspar is valued for its low iron and refractory mineral content, a low cost per unit of Al₂O₃, no volatiles and no waste.'Industrial Minerals & Rocks – Commodities, Markets and Uses' J. E. Kogel. Society for Mining, Metallurgy and Exploration, 2006. p. 458

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: Feldspars are used in the ceramic industry as a

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...

to form a glassy phase in bodies during firing, and thus promote vitrification. They are also used as a source of alkalies and alumina in glazes. The composition of feldspar used in different ceramic formulations varies depending on various factors, including the properties of the individual grade, the other raw materials and the requirements of the finished products. However, typical additions include: tableware, 15 % to 30 % feldspar; high-tension electrical porcelains, 25 % to 35 %; sanitaryware, 25 %; wall tile, 0 % to 10 %; and dental porcelain up to 80 % feldspar.

Earth science Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...

s: In earth sciences and archaeology, feldspars are used for potassium-argon dating, argon-argon dating and

luminescence dating Luminescence dating refers to a group of chronological dating methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating. It is useful to geologists and archaeologists who want to know when such an event ...

. Minor use: Some household cleaners (such as Bar Keepers Friend and Bon Ami) use feldspar to give a mild abrasive action.

Production

The

USGS The United States Geological Survey (USGS), founded as the Geological Survey, is an government agency, agency of the United States Department of the Interior, U.S. Department of the Interior whose work spans the disciplines of biology, geograp ...

estimated global production of feldspar in 2020 to be 26 million tonnes, with the top four producing countries being: China 2 million tonnes; India 5 million tonnes; Italy 4 million; Turkey 7.6 million tonnes.

Commercial grades

Typical mineralogical and chemical analyses of three commercial grades used in ceramics are:

Extraterrestrial

In October 2012, the ''Curiosity'' rover found high feldspar content in a Mars rock.Nasa's Curiosity rover finds 'unusual rock'
(12 October 2012)

BBC News BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broad ...

Gallery

File:Feldspar-Group-170604.jpg, Specimen of rare plumbian (

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

-rich) feldspar File:Beryl-Schorl-Feldspar-Group-288077.jpg, Crystallized white feldspar, with an upright 4 cm aquamarine crystal perched on it File:Feldspar-Group-Moonstone-36971.jpg, Feldspar and moonstone, from Sonora, Mexico File:Schorl-Feldspar-Group-49985.jpg, A cluster of euhedral feldspar crystals with a schorl crystal File:PIA16217-MarsCuriosityRover-1stXRayView-20121017.jpg, First X-ray view of Martian soil—feldspar, pyroxenes,

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

revealed (

Curiosity rover ''Curiosity'' is a car-sized Mars rover Space exploration, exploring Gale (crater), Gale crater and Mount Sharp on Mars as part of NASA's Mars Science Laboratory (MSL) mission. ''Curiosity'' was launched from Cape Canaveral Space Force Station ...

at " Rocknest", October 17, 2012) File:Lunar Ferroan Anorthosite (60025).jpg, Lunar ferrous

#60025 (

feldspar). Collected by

Apollo 16 Apollo 16 (April 1627, 1972) was the tenth human spaceflight, crewed mission in the United States Apollo program, Apollo space program, administered by NASA, and the fifth and penultimate to Moon landing, land on the Moon. It was the second o ...

from the Lunar Highlands near Descartes Crater. This sample is currently on display at the

National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...

Washington, D.C. Washington, D.C., formally the District of Columbia and commonly known as Washington or D.C., is the capital city and federal district of the United States. The city is on the Potomac River, across from Virginia, and shares land borders with ...

References

External links

* * {{Authority control Tectosilicates Triclinic minerals Monoclinic minerals Industrial minerals

Etymology

Compositions

Alkali feldspars

Ammonium feldspar

Barium feldspars

Plagioclase feldspars

Structure

Weathering

Applications

Production

Commercial grades

Extraterrestrial

Gallery

See also

References

Further reading

External links