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 " Eels " 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 . The 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 mono-clinic 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 900, whence the name orthoclase applied to these minerals; whilst in the anorthic or asymmetric felspars the corresponding angle is never exactly 900, 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 (KAlSi3O8) and hyalophane, a rare felspar containing barium (K2BaA14Si8O24)• The anorthic series includes at one end the soda-felspar albite (NaAlSi3O8) and at the other extremity the lime-felspar 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 oligoclase, andesine, labradorite and bytownite . There are also placed in the anorthic class a potash-felspar 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 feispar is that of pure orthoclase in the manufacture of artificial teeth . Felspar readily suffers chemical alteration, yielding kaolin (q.v.) . 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, LAB RADORITE,MIC ROCLINE, MOONSTONE,OLIGOCLASE,ORTHOCLASE, PLAGIOCLASE, SUN-STONE .