:''This article is about a mineral. For the ''Malcolm in the Middle'' character see list of characters''.  Compositional phase diagram of the different minerals that constitute the feldspar solid solution. }} '''Feldspars''' (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.Feldspar. [ What is Feldspar?] Industrial Minerals Association. Retrieved on July 18, 2007.  Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.&quot;Metamorphic Rocks.&quot; [ Metamorphic Rocks Information]. Retrieved on July 18, 2007 Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.Blatt, Harvey and Robert J. Tracy, ''Petrology,'' Freeman, 2nd ed., 1996, pp. 206–210 ISBN 0-7167-2438-3 Feldspars are also found in many types of sedimentary rock.&quot;Weathering and Sedimentary Rocks.&quot; [ Geology.] Retrieved on July 18, 2007.  ==Etymology==  Feldspar is derived from the German ''Feld'', &quot;field&quot;, and ''Spath'', &quot;a rock that does not contain ore&quot;. &quot;Feldspathic&quot; refers to materials that contain feldspar. The alternate spelling, ''felspar'', has now largely fallen out of use.  ==Compositions==  This group of minerals consists of framework  tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:  Potassium-Feldspar (K-spar) endmember KAlSi3O8  Albite endmember NaAlSi3O8  Anorthite endmember CaAl2Si2O8  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. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar. &quot;Sanidine Feldspar.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008. orthoclase (monoclinic),&quot;The Mineral Orthoclase.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008. and microcline (triclinic)&quot;Microcline Feldspar.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008. refer to polymorphs of K-feldspar. --&gt;  ===Alkali feldspars=== The alkali feldspars are as follows: * orthoclase (monoclinic),&quot;The Mineral Orthoclase.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008. — KAlSi3O8 * sanidine (monoclinic)&quot;Sanidine Feldspar.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008.  —(K,Na)AlSi3O8 *microcline (triclinic)&quot;Microcline Feldspar.&quot; [ Feldspar] Amethyst Galleries, Inc. Retrieved on February 8, 2008. — KAlSi3O8 *anorthoclase (triclinic) — (Na,K)AlSi3O8  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.Ralph, Jolyon &amp; Ida. &quot;Perthite.&quot; [ Perthite Profile on mindat.org] Retrieved on February 8, 2008. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.  ===Plagioclase feldspars===  The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses): * albite  (0 to 10) — NaAlSi3O8 * oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8 * andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8 * labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8 * bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8 * anorthite (90 to 100) — CaAl2Si2O8  Intermediate compositions of plagioclase feldspar also may 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 solution are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.  ===Barium feldspars=== The barium feldspars are monoclinic and comprise the following: * celsian — BaAl2Si2O8 * hyalophane — (K,Na,Ba)(Al,Si)4O8  Feldspars can form clay minerals through chemical weathering.  Feldspar output in 2005. ==Production and uses== 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..]] [[Labradorite.]] About 20 million tonnes of feldspar has been produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt) and China (2 Mt).[ Feldspar], USGS Mineral Commodity Summaries 2011  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 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 fiber. Pottery (including electrical insulators, sanitaryware, tableware, and tile) and other uses, such as fillers, accounted for the remainder.Lori E Apodaca [ Feldspar and nepheline syenite], USGS 2008 Minerals Yearbook  In in earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, thermoluminescence dating and optical dating.  