Perthites are common, whereas antiperthites are relatively rare. [6] Feldspars are also found in many types of sedimentary rocks.[7]. 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). [13] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope. The formula actually is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify it there. As is apparent from the preceding statements, solid solution plays an important role in the rock-making feldspars. The names of members of the series designate relative proportions of the end-members. Albite (NaAlSi3O8—usually designated Ab) and anorthite (CaAl2Si2O8—An) are end-members of the plagioclase series. Premium Membership is now 50% off. The plagioclase (Ab-An) series exhibits essentially complete solid solution at both high and low temperatures; coupled substitution of sodium and silicon by calcium and aluminum occurs. Some perthites, however, appear to have been formed as a result of partial replacement of original potassium feldspars by sodium-bearing fluids. The alkali (Or-Ab) series exhibits complete solid solution at high temperatures but only incomplete solid solution at low temperatures; substitution of potassium for sodium is involved. Perthite consists of intimate intermixtures of a potassium feldspar—either microcline or orthoclase—and a sodium-rich plagioclase that occurs as microscopic to macroscopic masses within the potassium feldspar host. Sanidine, anorthoclase, and the perthites are alkali feldspars whose chemical compositions lie between Or and Ab. In earth sciences and archaeology, feldspars are used for potassium-argon dating, argon-argon dating, and luminescence dating. 446–49 (Fig. Chemical weathering of feldspars results in the formation of clay minerals[20] such as illite and kaolinite. 11-95, "Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline Ignimbrite A', Gran Canaria, Canary Islands", "Visiting Our Past: Feldspar mining and racial tensions", Nasa's Curiosity rover finds 'unusual rock', "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals", https://en.wikipedia.org/w/index.php?title=Feldspar&oldid=991092208, Short description is different from Wikidata, Wikipedia articles incorporating text from the United States Geological Survey, Creative Commons Attribution-ShareAlike License, This page was last edited on 28 November 2020, at 06:03. With further cooling, in some cases the first and subsequently formed crystals will react continuously with the remaining liquid, thereby maintaining equilibrium; when the liquid becomes totally crystallized, the system will consist of homogeneous plagioclase crystals. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. [2][3], Feldspars crystallize from magma as both intrusive and extrusive igneous rocks[4] and are also present in many types of metamorphic rock. As indicated, sanidine and anorthoclase are high-temperature alkali feldspars, and perthite is their low-temperature analogue. They account for an estimated 60% of exposed rocks, as well as soils, clays, and other unconsolidated sediments, and are principal components in rock classification scheme… Several other elements also have been recorded as traces in feldspar analyses; it seems very likely, however, that some of these elements may reside in impurities—i.e., within unrecognized microscopic or submicroscopic inclusions of other minerals. Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight. In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content. The minerals of the plagioclase series are listed in the table below along with their relative abundance of albite (Ab) and anorthite (An). The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75). [5] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. [9][10] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. Barium feldspars are also considered alkali feldspars. (Members of solid-solution series are single crystalline phases whose chemical compositions are intermediate to those of two or more end-members.) Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. There is a different chemical formula for each of the three major types of feldspar, varying slightly due to the elements within the mineral.... See full answer below. Feldspars are naturally occurring substances, so the ratio between the potash (K 2 O) and the soda (Na 2 O) will vary somewhat. This sample is currently on display at the National Museum of Natural History in Washington, D.C. A group of rock-forming tectosilicate minerals, Gemmological classifications by E. Ya. The alternate spelling, felspar, has fallen out of use. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. This affects the properties of the feldspar, in that the soda tends to … The feldspars are mixtures of potassium alumino-silicate (K 2 O Al 2 O 3 6SiO 2) and sodium alumino-silicate, also known as albite (Na 2 O Al 2 O 3 6SiO 2). By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Learn more about the properties and uses of feldspar … The first of these include: 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. [21], Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber.