Feldspar

Template:Short description {{#invoke:other uses|otheruses}} Template:Use British English Template:Stack

Feldspar (Template:IPAc-en Template:Respell; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium.<ref name="Glossary2005">Template:Cite book</ref> The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars.<ref name="DeerHowieZussman2001">Template:Cite book</ref> Feldspars make up about 60% of the Earth's crust<ref name="Glossary2005"/> and 41% of the Earth's continental crust by weight.<ref>Template:Cite book</ref><ref name="Rudnick2003">Template:Cite book</ref>

Feldspars crystallize from magma as both intrusive and extrusive igneous rocks<ref>Template:Cite journal</ref> and are also present in many types of metamorphic rock.<ref>"Metamorphic Rocks." Metamorphic Rocks Information Template:Webarchive. Retrieved on July 18, 2007</ref> Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.<ref>Blatt, Harvey and Tracy, Robert J. (1996) Petrology, Freeman, 2nd ed., pp. 206–210 Template:ISBN</ref> Feldspars are also found in many types of sedimentary rocks.<ref>"Weathering and Sedimentary Rocks." Geology. Template:Webarchive Retrieved on July 18, 2007.</ref>

EtymologyEdit

The name feldspar derives from the German {{#invoke:Lang|lang}}, a compound of the words {{#invoke:Lang|lang}} ("field") and {{#invoke:Lang|lang}} ("flake"). {{#invoke:Lang|lang}} had long been used as the word for "a rock easily cleaved into flakes"; {{#invoke:Lang|lang}} 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).<ref name="Lüschen">Template:Citation</ref> The change from {{#invoke:Lang|lang}} to -spar was influenced by the English word spar,<ref>Template:OEtymD</ref> meaning a non-opaque mineral with good cleavage.<ref name="OED1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 feldspar and silica, unrelated to the obsolete spelling "felspar".

CompositionsEdit

File:Feldspar group.svg

Compositional phase diagram of the different minerals that constitute the feldspar solid solution

The feldspar group of minerals consists of tectosilicates, 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 endmember K Al Si₃O₈<ref name="feldspar">Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007.</ref>
albite endmember NaAlSi₃O₈<ref name = "feldspar"/>
anorthite endmember CaAl₂Si₂O₈<ref name = "feldspar"/>

Solid solutions between orthoclase and albite are called alkali feldspar.<ref name = "feldspar"/> Solid solutions between albite and anorthite are called plagioclase,<ref name="feldspar" /> 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, is the basis for the QAPF classification of igneous rock.<ref name="iugs-1991">Template:Cite journal</ref><ref name="BGS-1999">Template:Cite journal</ref><ref name="philpotts-ague-2009-139-143">Template:Cite book</ref> 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 magma.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>

Alkali feldsparsEdit

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:

orthoclase (monoclinic)<ref name="orthoclase">"The Mineral Orthoclase". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> Template:Chem2
sanidine (monoclinic)<ref name="sanidine">"Sanidine Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> Template:Chem2
microcline (triclinic)<ref name="microcline">"Microcline Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> Template:Chem2
anorthoclase (triclinic) Template:Chem2

Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.Template:Sfn Sanidine is stable at the highest temperatures, and microcline at the lowest.<ref name="orthoclase" /><ref name="sanidine" /> Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.<ref>Ralph, Jolyon and Chou, Ida. "Perthite". Perthite Profile on mindat.org. Retrieved on February 8, 2008.</ref> Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Ammonium feldsparEdit

Buddingtonite is an ammonium feldspar with the chemical formula: NH₄AlSi₃O₈.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.

Barium feldsparsEdit

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,<ref name="DeerHowieZussman2001"/> and sometimes they are classified as a sub-group of alkali feldspars.<ref name="MindatGroup">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The barium feldspars are monoclinic and include the following:

celsian Template:Chem2<ref>Celsian–orthoclase series on Mindat.org.</ref>
hyalophane Template:Chem2<ref>Celsian–hyalophane series on Mindat.org.</ref>

Plagioclase feldsparsEdit

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

Intermediate compositions of exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in plagioclase solid solutions are more complex than those in alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75).

StructureEdit

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 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.Template:Sfn

Anorthite crankshaft.png

Diagram showing part of a crankshaft chain of feldspar
Anorthite c.png

Feldspar crystal structure viewed along the c axis
Anorthite a.png

Feldspar crystal structure viewed along the a axis
Anorthite b.png

Feldspar crystal structure viewed along the b axis

WeatheringEdit

Chemical weathering of feldspars happens by hydrolysis and produces clay minerals, including illite, smectite, and kaolinite. Hydrolysis of feldspars begins with the feldspar dissolving in water, which happens best in acidic or basic solutions and less well in neutral ones.<ref name=":0">Template:Citation</ref> The speed at which feldspars are weathered is controlled by how quickly they are dissolved.<ref name=":0" /> 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.<ref>Template:Cite book</ref> About 40% of minerals in sedimentary rocks are clays and clays are the dominant minerals in the most common sedimentary rocks, mudrocks.<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> They are also an important component of soils.<ref name=":1" /> Feldspar that has been replaced by clay looks chalky compared to more crystalline and glassy unweathered feldspar grains.<ref>Template:Cite book</ref>

Feldspars, especially plagioclase feldspars, are not very stable at the Earth's surface due to their high formation temperature.<ref name=":1" /> 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.<ref name=":2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Sandstones with large amounts of feldspar are called arkoses.<ref name=":2" />

ApplicationsEdit

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.<ref name=usgs>Apodaca, Lori E. (2008). Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook</ref>

Glass: 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.<ref name="auto">'Industrial Minerals & Rocks – Commodities, Markets and Uses' J. E. Kogel. Society for Mining, Metallurgy and Exploration, 2006. p. 458</ref>

Ceramics: Feldspars are used in the ceramic industry as a flux 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.<ref name="auto"/> 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.<ref>Singer, F. and Singer, S.S. (1971). 'Industrial Ceramics'. Chapman & Hall.</ref>

Earth sciences: In earth sciences and archaeology, feldspars are used for potassium-argon dating, argon-argon dating and luminescence dating.

Minor use: Some household cleaners (such as Bar Keepers Friend and Bon Ami) use feldspar to give a mild abrasive action.<ref>Template:Cite news</ref>

ProductionEdit

The USGS 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.<ref>Feldspar and Nepheline Syenite. U.S. Geological Survey, Mineral Commodity Summaries, January 2020</ref>

Commercial gradesEdit

Typical mineralogical and chemical analyses of three commercial grades used in ceramics are:<ref>Sugden, A. (2004). 'Feldspar Facts'. Ceramic Review, Issue 207, May/June 2004.</ref>

Product name	Norfloat K	Forshammar	FFF K6
Country	Norway	Sweden	Finland
Producing company	North Cape	Template:Ill	Sibelco

Albite, %	23	40	41
Microcline, %	71	23	37
Anorthite, %	3	—	4
Quartz, %	3	33	8

SiO₂, %	65.9	75.7	67.9
Al₂O₃, %	18.6	14.1	18.3
Fe₂O₃, %	0.07	0.15	0.11
TiO₂, %	—	0.02	0.01
CaO, %	0.40	0.30	0.70
MgO, %	—	0.10	0.01
K₂O, %	11.8	3.8	6.4
Na₂O, %	2.9	5.0	5.5
LOI, %	0.2	0.5	0.2

ExtraterrestrialEdit

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

GalleryEdit

Feldspar-Group-170604.jpg

Specimen of rare plumbian (lead-rich) feldspar
Beryl-Schorl-Feldspar-Group-288077.jpg

Crystallized white feldspar, with an upright 4 cm aquamarine crystal perched on it
Feldspar-Group-Moonstone-36971.jpg

Feldspar and moonstone, from Sonora, Mexico
Schorl-Feldspar-Group-49985.jpg

A cluster of euhedral feldspar crystals with a schorl crystal
PIA16217-MarsCuriosityRover-1stXRayView-20121017.jpg

CitationClass=web }}</ref>
Lunar Ferroan Anorthosite (60025).jpg

Lunar ferrous anorthosite #60025 (plagioclase feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, D.C.

ReferencesEdit

Template:USGS

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External linksEdit

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