Felspar or Feldspar

FELSPAR or FELDSPAR, a name applied to a group of minerals of great importance as rock constituents. The name "feldspar" was used by Wallerius in his Mineralogy (1747) in the Swedish form feltspat, meaning fieldspar, but it was subsequently written "felspar" by R. Kirwan, who erroneously assumed the derivation of the name from fels, a rock. The corrupt form felspar is now in general use, but in German the old name is preserved it feldspat. In composition the felspars are alumino-silicates of potassium, sodium, calcium and barium. A strontium felspar has been artificially prepared but is unknown in nature. According to the symmetry of their crystals, the felspars are divisible into two series, a monoclinic series and a triclinic series.

Monoclinic Series Orthoclase {SAadnujidairniae = Or.

(KAI

with some solid solution with NaAl Celsian (Ba = Ce.

Hyalophane (solid solution of K M

with 13a, Triclinic Series Albite (Na Al = Ab.

Oligoclase Plagioclase Andesine Solid solutions of Labradorite Ab and An.

Bytownite Anorthite = An.

Microcline (KAI

Anorthoclase (Cryptoperthite) (Solid solution of KAI and Na Al All felspars show a similarity of crystal form, being often pris matic but flattened parallel to the face (oio), or elongated parallel to the axis a with small prism faces. Anorthoclase has a charac teristic habit, the crystals being elongated parallel to the vertical axis, only the faces 1 i o, i >'o and ao i being developed. All felspars cleave readily parallel to ooi and oio, these making angles of 9o° in the monoclinic varieties and oblique angles in the triclinic mem bers. Twinned crystals of felspar are common and the twinning serves to distinguish various members of the group both in hand specimens and by optical methods under the microscope. In the monoclinic series the commonest twin laws are the Carlsbad axis c), the Manebach (twin plane ooi) and the Baveno plane 021). In the triclinic series, in addition to these three laws, multiple twinning of the albite (twin plane (io) and (twin axis b) types is very common. These twins are microscopi cally recognized by the fine striations seen on the cleavage planes ooI (albite and pericline) oio (pericline).

Pure felspars are colourless, but owing to the presence of minute

inclusions or the development of alteration products, most natural felspars are white, pink, yellowish or greenish. Some are used as ornamental stones on account of their colour (see AMAZON STONE) others are prized for their pearly opalescence (see MOON