1911 Encyclopædia Britannica/Felspar

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 fältspat.

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 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 (KAlSi₃O₈) and hyalophane, a rare felspar containing barium (K₂BaAl₄Si₈O₂₄). 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 felspar 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, Labradorite, Microcline, Moonstone, Oligoclase, Orthoclase, Plagioclase, Sun-stone.