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Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.[1]
Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.[2] Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite.[3] Feldspars are also found in many types of sedimentary rock.[4]
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Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.[5]
This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
Potassium-Feldspar (K-spar) endmember KAlSi3O8[1]
Anorthite endmember CaAl2Si2O8[1]
Solid solutions between K-feldspar and albite are called alkali feldspar.[1] Solid solutions between albite and anorthite are called plagioclase,[1] 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.
The alkali feldspars are as follows:
Sanidine is stable at the highest temperatures, and microcline at the lowest.[7][6] 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.[9] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):
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 colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
The barium feldspars are monoclinic and comprise the following:
Feldspars can form clay minerals through chemical weathering.[10]
In 2005, Italy was the top producer of feldspar with almost one fifth of world share, followed by Turkey, China and Thailand—reports the International Monetary Fund.
FELSPAR, or Feldspar, a name applied to a group of mineral silicates of much importance as rock-constituents. The name, taken from the Ger. Feldspath, was originally written with a " d " but in 1794 it was written " felspar " by R. Kirwan, on the assumption that it denoted a mineral of the " fels " rather than of the " field," and this corrupted form is now in common use in England. By some of the earlier mineralogists it was written feltspar," from the Swedish form fdltspat.
I he felspar - group is divided into two subgroups according to the symmetry of the crystals. Although the crystals of all felspars present a general resemblance in habit, they are usually regarded as belonging to two systems, some felspars being monoclinic and others anorthic. Figures of the crystals are given in the articles on the different species. Two cleavages are generally well marked. In the monoclinic or monosymmetric felspars these, being parallel to the basal pinacoid and clinopinacoid, necessarily make an angle of 90°, whence the name orthoclase applied to these minerals; whilst in the anorthic or asymmetric felspars the corresponding angle is never exactly 90°, and from this obliquity of the principal cleavages they are termed plagioclase (see Orthoclase and Plagioclase). There are consequently two series of felspars, one termed orthoclastic or orthotomous, and the other plagioclastic or clinotomous.
F. E. Mallard suggested that all felspars are really asymmetric, and that orthoclase presents only a pseudo-monosymmetric habit, due to twinning. Twin-crystals are very common in all the felspars, as explained under their respective headings.
The two divisions of the felspar-group founded on differences of crystalline symmetry are subdivided according to chemical composition. All the felspars are silicates containing aluminium with some other metallic base or bases, generally potassium, sodium or calcium, rarely barium, but never magnesium or iron. The monoclinic series includes common potash-felspar or orthoclase (KA1Si 3 O 8) and hyalophane, a rare felspar containing barium (K 2 BaAl 4 Si 8 O 24). The anorthic series includes at one end the soda-felspar albite (NaA1Si 3 O 8) and at the other extremity the lime-felspar anorthite (CaAl 2 Si 2 O 8). It was suggested by G. Tschermak in 1864 that the other plagioclastic felspars are isomorphous mixtures in various proportion of albite (Ab) and anorthite (An). These intermediate members are the lime-soda felspars known as oligoclase, andesine, labradorite and bytownite. There are also placed in the anorthic class a potashfelspar called microcline, and a rare soda-potash-felspar known as anorthoclase.
The specific gravity of the felspars has been shown by G. Tschermak and V. Goldschmidt to vary according to their chemical composition, rising steadily from 2.57 in orthoclase to 2.75 in anorthite. All the felspars have a hardness of 6 to 6.5, being therefore rather less hard than quartz. Pure felspar is colourless, but the mineral is usually white, yellow, red or green. Certain felspars are used as ornamental stones on account of their colour (see Amazon Stone). Other felspars are prized for their pearly opalescence (see Moonstone), or for their play of iridescent colours (see Labradorite), or for their spangled appearance, like aventurine (see SUN-Stone).
Felspar is much used in the manufacture of porcelain by reason of its fusibility. In England the material employed is mostly orthoclase from Scandinavia, often known as " Swedish spar." The high translucency of " ivory porcelain " depends on the large proportion of felspar in the body. The mineral is also an important constituent of most ceramic glazes. The melting points of felspars have been investigated by Prof. J. Joly, Prof. C. A. Doelter y Cisterich and especially by A. L. Day and E. T. Allen in the Geophysical Laboratory of the Carnegie Institute at Washington.
Among the applications of felspar is that of pure orthoclase in the manufacture of artificial teeth.
Felspar readily suffers chemical alteration, yielding kaolin. The turbidity of orthoclase is usually due to partial kaolinization. Secondary mica is also a common result of alteration, and among other products are pinite, epidote, saussurite, chlorite, wollastonite and various zeolites.
See Albite, Amazon Stone, Andesine, Anorthite, Bytownite, Labradorite,Microcline,Moonstone,Oligoclase,Orthoclase, Plagioclase, Sun-Stone.
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