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FELSPAR, or FELDSPAR

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Originally appearing in Volume V10, Page 245 of the 1911 Encyclopedia Britannica.
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FELSPAR, or FELDSPAR , a name applied to a See also:group of See also:mineral silicates of much importance as See also:rock-constituents. The name, taken from the Ger. Feldspath, was originally written with a " d " but in 1794 it was written " felspar " by R. See also:Kirwan, on the See also:assumption that it denoted a mineral of the " Eels " rather than of the " See also:field," and this corrupted See also:form is now in See also:common use in See also:England. By some of the earlier mineralogists it was written " feltspar," from the See also:Swedish form fdltspat. The felspar - group is divided into two subgroups according to the symmetry of the crystals. Although the crystals of all felspars See also:present a See also:general resemblance in See also:habit, they are usually regarded as belonging to two systems, some felspars being mono-clinic and others anorthic. Figures of the crystals are given in the articles on the different See also:species. Two cleavages are generally well marked. In the See also:monoclinic or monosymmetric felspars these, being parallel to the basal pinacoid and clinopinacoid, necessarily make an See also:angle of 900, whence the name See also:orthoclase applied to these minerals; whilst in the anorthic or See also:asymmetric felspars the corresponding angle is never exactly 900, and from this obliquity of the See also:principal cleavages they are termed See also:plagioclase (see ORTHOCLASE and PLAGIOCLASE). There are consequently two See also: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 See also:

differences of crystalline symmetry are subdivided according to chemical See also:composition. All the felspars are silicates containing See also:aluminium with some other metallic See also:base or bases, generally See also:potassium, See also:sodium or See also:calcium, rarely See also:barium, but never See also:magnesium or See also:iron. The monoclinic series includes common potash-felspar or orthoclase (KAlSi3O8) and hyalophane, a rare felspar containing barium (K2BaA14Si8O24)• The anorthic series includes at one end the soda-felspar See also:albite (NaAlSi3O8) and at the other extremity the See also:lime-felspar See also:anorthite (CaAl2Si2O8). 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 See also:oligoclase, See also:andesine, See also:labradorite and See also:bytownite. There are also placed in the anorthic class a potash-felspar called See also:microcline, and a rare soda-potash-felspar known as anorthoclase. The specific gravity of the felspars has been shown by G. Tschermak and V.

See also:

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 See also:quartz. Pure felspar is colourless, but the mineral is usually See also:white, yellow, red or See also:green. Certain felspars are used as ornamental stones on See also:account of their See also:colour (see See also:AMAZON.See also:STONE). Other felspars are prized for their pearly opalescence (see See also:MOONSTONE), or for their See also:play of iridescent See also:colours (see LABRADORITE), or for their spangled See also:appearance, like See also:aventurine (see See also:SUN-STONE). Felspar is much used in the manufacture of See also:porcelain by See also:reason of its fusibility. In England the material employed is mostly orthoclase from Scandinavia, often known as " Swedish spar." The high translucency of " See also:ivory porcelain " depends on the large proportion of felspar in the See also: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. See also:

Day and E. T. See also:Allen in the Geophysical Laboratory of the See also:Carnegie See also:Institute at See also:Washington. Among the applications of feispar is that of pure orthoclase in the manufacture of artificial See also:teeth. Felspar readily suffers chemical alteration, yielding See also:kaolin (q.v.). The turbidity of orthoclase is usually due to partial kaolinization. Secondary See also:mica is also a common result of alteration, and among other products are pinite, See also:epidote, saussurite, See also:chlorite, See also:wollastonite and various See also:zeolites. See ALBITE, AMAZON STONE, ANDESINE, ANORTHITE, BYTOWNITE, LAB RADORITE,MIC ROCLINE, MOONSTONE,OLIGOCLASE,ORTHOCLASE, PLAGIOCLASE, SUN-STONE.

End of Article: FELSPAR, or FELDSPAR

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