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Feldspars () are a group of rock-forming
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, silica (silicon dioxide) SiO2 is usually cons ...
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
s that make up about 41% of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. Generally identified by con ...
's
continental crust 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneous rock, igneous, Sedimentary rock, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallo ...
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, macrosco ...
lize from
magma 300px, Lava flow on Hawaii (island), Hawaii. Lava is the extrusive equivalent of magma. Magma (from Ancient Greek μάγμα (''mágma'') meaning "thick unguent") is the molten or semi-molten natural material from which all igneous rocks are form ...
as both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of
sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at the Earth's surface, followed by Cementation (geology), cementation. Sedimentation is the coll ...
s.


Compositions

This 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, silica (silicon dioxide) SiO2 is usually cons ...
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: * potassium feldspar (K-spar) endmember
K
K
Al
Si
Si
Oxygen, OFeldspar
What is Feldspar?
Industrial Minerals Association. Retrieved on July 18, 2007.
* albite endmember Sodium, NaAlSiO * anorthite endmember Calcium, CaAlSiO Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, 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 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. Calcium-rich plagioclase is the first feldspar to crystallize from a cooling magma, but 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.


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 include: * orthoclase (monoclinic)"The Mineral Orthoclase"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
* sanidine (monoclinic)"Sanidine Feldspar"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
* microcline (triclinic)"Microcline Feldspar"
Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
* anorthoclase (triclinic) Potassium and sodium feldspars are not perfectly Miscibility, miscible 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 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. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.


Barium feldspars

Barium feldspars are also considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. The barium feldspars are monoclinic and include the following: * celsian * hyalophane


Plagioclase feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses): * albite (0 to 10) * oligoclase (10 to 30) * andesine (30 to 50) * labradorite (50 to 70) * bytownite (70 to 90) * anorthite (90 to 100) Intermediate compositions of plagioclase feldspar also may Exsolution, 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 the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution Lamellar structure, lamellae known as Bøggild intergrowth. The specific gravity in the plagioclase series increases from albite (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 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 the structure 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


Etymology

The name ''feldspar'' derives from the German language, German ''Feldspat'', a compound of the words ''Feld'' ("field") and ''Spat'' ("flake"). ''Spat'' had long been used as the word for "a Rock (geology), rock easily cleaved into flakes"; ''Feldspat'' 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 ''Spat'' to ''-spar'' was influenced by the English word Spar (mineralogy), ''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 redundant spelling 'felspar'.


Weathering

Weathering, Chemical weathering of feldspars happens by hydrolysis and produces Clay mineral, 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. 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 Ion, ions in solution, with the variety of ion controlled by the type of feldspar reacting. The abundance of feldspars in the Earth's Crust (geology), crust means that clays are very abundant weathering products. About 40% of Mineral, minerals in Sedimentary rock, sedimentary rocks are clays, and clays are the dominant minerals in the most common sedimentary rocks, Mudrock, mudrocks. They are also an important component of Soil, soils. 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 Sediment transport, transported a short distance in cold and/or dry conditions that didn't promote weathering, and that it was quickly buried by other sediment. Sandstones with large amounts of feldspar are called Arkose, arkoses.


Production and uses

About 20 million tonnes of feldspar were produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt). Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the Pottery firing, firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fibre. Ceramics (including electrical insulators, sanitary ware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.Apodaca, Lori E
Feldspar and nepheline syenite
USGS 2008 Minerals Yearbook
Bon Ami, which had a mine near Little Switzerland, North Carolina, used feldspar as an abrasive in its cleaners. The Little Switzerland Business Association says the McKinney Mine was the largest feldspar mine in the world, and North Carolina was the largest producer. Feldspar had been discarded in the process of mining mica until William Dibbell sent a premium quality product to the Ohio company Golding and Sons around 1910. In earth sciences and archaeology, feldspars are used for potassium-argon dating, argon-argon dating, and luminescence dating. In October 2012, the Mars Curiosity rover analysed a rock that turned out to have a high feldspar content.Nasa's Curiosity rover finds 'unusual rock'
(12 October 2012) BBC News.


Images

File:Feldspar-Group-170604.jpg, Specimen of rare plumbian (lead-rich) feldspar File:Beryl-Schorl-Feldspar-Group-288077.jpg, Perched on crystallized, white feldspar is an upright 4 cm Aquamarine (gemstone), aquamarine crystal File:Feldspar-Group-Moonstone-36971.jpg, Feldspar and moonstone, from Sonora, Mexico File:Schorl-Feldspar-Group-49985.jpg, Schorl crystal on a cluster of euhedral feldspar crystals File:PIA16217-MarsCuriosityRover-1stXRayView-20121017.jpg, First X-ray crystallography#Mineralogy and metallurgy, X-ray view of Martian soil—feldspar, pyroxenes, olivine revealed (Curiosity rover at "Rocknest (Mars), Rocknest", October 17, 2012). File:Lunar Ferroan Anorthosite (60025).jpg, Moon, Lunar ferrous anorthosite #60025 ( plagioclase feldspar). Collected by Apollo 16 from the Geology of the Moon#Highlands and craters, Lunar Highlands near Descartes (crater), Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, D.C.


See also

* * *Rainbow lattice sunstone


References

*


Further reading

* Bonewitz, Ronald Louis (2005). ''Rock and Gem''. New York: DK Publishing. .


External links

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