Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. They are used in the manufacture of glass and ceramics; transparent, highly coloured, or iridescent varieties are sometimes used as gemstones. [19] Feldspathic refers to materials that contain feldspar. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: Solid solutions between K-feldspar and albite are called alkali feldspar. Orthoclase (Monoclinic) KAlSi3O8, 2. In addition, peristerite is the name given to feldspar containing approximately equal amounts of intergrown alkali feldspar and plagioclase.[14]. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75). (1996), Klein, Cornelis and Cornelius S. Hurlbut, Jr. Handbook of Mineralogy, Wiley, pp. Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. [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. Barium feldspars are also considered alkali feldspars. Synonym of: Anorthoclase, Microcline, Orthoclase, Perthite, Sanidine, Sodic Albite Feldspar Group minerals rich in Potassium or Sodium. The alkali feldspars are as follows: 1. Omissions? The change from Spat to -spar was influenced by the English word spar,[18] meaning a non-opaque mineral with good cleavage. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. Feldspathoids also occur in a few other kinds of rocks—. Most range from pinpoint to nut size, but some are as much as several feet in diameter. Premium Membership is now 50% off! [12] Sanidine is stable at the highest temperatures, and microcline at the lowest. [17] The name feldspar derives from the German Feldspat, a compound of the words Feld ("field") and Spat ("flake"). In the Earth’s crust the alkali feldspars display a range of ordering from the fully random distribution of sanidine and orthoclase to the fully ordered distribution of microcline. The major third component for plagioclase feldspars is Or. Let us know if you have suggestions to improve this article (requires login). The major rock-forming minerals in the Feldspar group include albite, anorthite, orthoclase and plagioclase. 446–49 (Fig. This group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Low-temperature forms include orthoclase, microcline, and adularia (all potassium aluminosilicates); these have an ordered arrangement of such atoms. The alternate spelling, felspar, has fallen out of use. Plagioclase feldspars are the most common rock forming mineral on earth and form the ground-mass of many mafic, felsic, and intermediate rock. 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. See feldspar. Sodium rich plagioclase or albite is present in more felsic rocks and is associated with alkali feldspar, mica, quartz, and many other minerals. 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. [6] Feldspars are also found in many types of sedimentary rocks.[7]. Kievlenko (1980), updated, Blatt, Harvey and Tracy, Robert J. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... …tiny fibres and plates of alkali feldspar radiating outward from a centre. 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. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder. The barium feldspars are monoclinic and include the following: The plagioclase feldspars are triclinic. Both the sodium and potassium aluminosilicates have several distinct forms, each form with a different structure. Feldspar is the name of a large group of rock-forming silicate minerals that make up over 50% of Earths crust. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. Anorthoclase(Triclinic) (Na,K)AlSi3O8. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Chemical weathering of feldspars results in the formation of clay minerals[20] such as illite and kaolinite. The term 'felsic', meaning light colored minerals such as quartz and feldspars, is an acronymic word derived from feldspar and silica, unrelated to the redundant spelling 'felspar'. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: The alkali feldspars are primarily important as constituents of rocks; they are very widespread and abundant in alkali and acidic igneous rocks (particularly syenites, granites, and granodiorites), in pegmatites, and in gneisses. Updates? The alkali feldspars may be regarded as mixtures of sodium aluminosilicate (NaAlSi 3 O 8) and potassium aluminosilicate (KAlSi 3 O 8). Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; and those where potassium is replaced by barium. The feldspar group is a fairly large group with nearly 20 members recognized, but only nine are well known and common. [9][10] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The relatively large ones tend to be internally complex and to contain concentric shells of feldspar fibres with or without accompanying…, …a major constituent of many alkalic igneous rocks such as nepheline syenites. 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. [2][3], Feldspars crystallize from magma as both intrusive and extrusive igneous rocks[4] and are also present in many types of metamorphic rock. [24], Specimen of rare plumbian (lead-rich) feldspar, Perched on crystallized, white feldspar is an upright 4 cm aquamarine crystal, Feldspar and moonstone, from Sonora, Mexico, Schorl crystal on a cluster of euhedral feldspar crystals, First X-ray view of Martian soil—feldspar, pyroxenes, olivine revealed (Curiosity rover at "Rocknest", October 17, 2012).[25]. Alkali feldspar, any of several common silicate minerals that often occur as variously coloured, glassy crystals. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. Albite is considered both a plagioclase and alkali feldspar. The perthitic textures in the alkali feldspars of many granites can be seen with the naked e… Feldspar is a group of silicate minerals and is collectively the most common mineral found on the surface of the Earth. In petrology the group is split into two major groups: Alkali feldspar - feldspar minerals rich in sodium or potassium. Thus, to show graphically the variation in chemical composition of the feldspars, a ternary composition diagram is needed with two independent composition axes and three end member components: Or, Ab, and An (see at- tached diagram). Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. [22], Bon Ami, which had a mine near Little Switzerland, North Carolina, used feldspar as an abrasive in its cleaners.

alkali feldspar group

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