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ORTHOCLASE

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Originally appearing in Volume V20, Page 333 of the 1911 Encyclopedia Britannica.

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ORTHOCLASE , an important See also:

rock-forming See also:

MINERAL

mineral belonging to the See also:

FELSPAR, or FELDSPAR

felspar See also:

GROUP

group (see FELSPAR). It is a potash-felspar, KAlSi303, and crystallizes in the See also:

MONOCLINIC

monoclinic See also:

SYSTEM

system. Large and distinctly See also:

DEVELOPED

developed crystals are frequently found in the drusy cavities of granites and pegmatites. Crystals differ somewhat in See also:

HABIT (through the French from Lat. habitus, from habere, to have, hold, or, in a reflective sense, to be in a certain condition; in many of the English senses the French use habitude, not habit)

habit; for example, they may be prismatic with an orthorhombic aspect (fig. I), as in the variety adularia (from the Adular Mountains in the St Gotthard region); or See also:

TABULAR

tabular (fig. 2), being flattened parallel to the clino-pinacoid or See also:

PLANE

plane of symmetry b (oro), as in the variety sanidine (aavis, vavlbos, a See also:

BOARD (O. Eng. bord)

board); or again the crystals may be elongated in the direction of the edge between b and the basal plane c (oo1), which is a characteristic habit of orthoclase from the See also:

GRANITE (adapted from the Ital. granite, grained; Lat. granum, grain)

granite quarries at See also:

BAVENO

Baveno in See also:

Italy. Twinning is frequent, and there are three well-defined twin-See also:

LAWS

laws: (1) See also:

CARLSBAD

Carlsbad twins (fig. 4). Here the two individuals of the twin interpenetrate or are See also:

UNITED

united parallel to the clino- pinacoid: one individual may be brought into the position of the other by a rotation of 18o° about the See also:

VERTICAL (from Lat. vertex, highest point)

vertical crystallographic See also:

AXIS (Lat. for " axle ")

axis or See also:

PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism-edge. Such twinned crystals are found at Carlsbad in Bohemia and many other places. (2) Baveno twins (fig. 5).

These twins, in which n (021) is the twin-plane, are See also:

COMMON

common at Baveno. (3) Manebach twins (fig. 6). The twin-plane here is c (oox); examples of this rarer twin were first found at Manebach in Thuringia. An important See also:

CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character of orthoclase is the cleavage. There is a direction of perfect cleavage parallel to the basal plane c, on which plane the lustre is consequently often pearly; and one less highly developed parallel to the plane of symmetry b. The See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle between these two cleavages is 900, hence the name See also:

CATHOLIC (Gr. KcaBoXuK6s, general, universal)

Catholic Apostolic Eastern See also:

Church "), the See also:

HISTORICAL

historical repreorthoclase (from the Gr. bpBos, right, and uxav, to break), given by A. Breithaupt in 1823, who was the first to distinguish orthoclase from the other felspars. There are also imperfect cleavages parallel to the faces of the prism m (rio). The hardness is 6, and the sp. gr. 2.56. Crystals are some-times colourless and transparent with a glassy aspect, as in the varieties adularia, sanidine and the rhyacolite of See also:

MONTE

Monte Somma, See also:

VESUVIUS (also Vesevus in ancient poets)

Vesuvius.

The See also:

OPTICAL

optical characters are somewhat variable, the plane of the optic axes being perpendicular to the plane of symmetry in some crystals and parallel to it in others: further, when some crystals are heated, the optic axes gradually See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change from one position to the other. In all cases, however, the acute negative See also:

BISECTRIX (fem. of Lat. bisector, from bi-, two, secare, to cut)

bisectrix of the optic axes lies in the plane of symmetry and is inclined to the edge b/c at 3-7°, or, in varieties See also:

rich in soda, at 10-12°. The mean refractive See also:

INDEX

index is 1.524, and the See also:

double See also:

refraction is weak (o•oo6). Analyses of orthoclase usually prove the presence of small amounts of soda and See also:

lime in addition to potash. These constituents are, however, probably See also:

PRESENT

present as See also:

PLAGIOCLASE

plagioclase (See also:

ALBITE

albite and See also:

OLIGOCLASE

oligoclase) intergrown with the orthoclase. The two minerals are interlaminated parallel to the ortho-pinacoid (Too) or the pinacoid (8or), and they may readily be distinguished in the flesh-red See also:

AVENTURINE, or AVANTURINE

aventurine-felspar, known as perthite, from See also:

Perth in See also:

LANARK

Lanark See also:

COUNTY (through Norm. Fr. comae, cf. O. Fr. cunte, conk', Mod. Fr. comae, from Lat. comitatus, cf. Ital. comitato, Prov. comtat; see COUNT)

county, See also:

Ontario. Frequently, however, as in microperthite and cryptoperthite, this is on a microscopic See also:

scale or so See also:

minute as to be no longer recognizable. These directions (Too)•and (8or) are planes of parting in orthoclase, and along them alteration frequently takes See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place, giving rise to See also:

SCHILLER, JOHANN CHRISTOPH FRIEDRICH VON (1759-18o5)

schiller effects. See also:

Moon-See also:

stone (q.v.) shows a pearly opalescent reflection on these planes; and brilliant coloured reflections in the same directions are exhibited by the labradorescent orthoclase from the See also:

AUGITE

augite-See also:

SYENITE

syenite of Fredriksvarn and See also:

LAURVIK, LARVIK

Laurvik in See also:

SOUTHERN

southern See also:

Norway, which is much used as an ornamental stone. The same effect is shown to a lesser degree by murchisonite, named in See also:

HONOUR (Lat. honos or lwnor, honoris; in English the word was spelled with or without the u indifferently until the 17th century, but during the 18th century it became fashion-able to spell the word " honor "; Johnson's and Webster's Dictionaries stereoty

honour of See also:

Sir R.I. See also:

MURCHISON, SIR RODERICK IMPEY (1792-1871)

Murchison, from the Triassic See also:

CONGLOMERATE (from the Lat. conglomerare, to form into a ball, glomus, glomeris; so also the general term " conglomeration " for a miscellaneous collection of things, gathered together in a mass)

conglomerate of Heavitree near See also:

Exeter. Orthoclase forms an essential constituent of many acidic igneous rocks (granite, syenite, See also:

PORPHYRY (IlopcPisptos) (A.D. 233-c. 304)

porphyry, See also:

TRACHYTE (Gr. rpaxus, rough)

trachyte, See also:

PHONOLITE (Gr. ~vi7, sound, and Moos, stone)

phonolite, &c.) and of crystalline See also:

SCHISTS (Gr. vxq'ecv, to split)

schists and gneisses.

In porphyries and in some granites (e.g. those of Shap in See also:

Westmorland, See also:

CORNWALL

Cornwall, &c.) it occurs as em-bedded crystals with well-defined outlines, but usually it presents no crystalline See also:

FORM (Lat. forma)

form. In the trachyte of the Drachenfels and the Laacher See in Rhenish See also:

Prussia there are large porphyritic crystals of glassy sanidine. The best crystals are those found in the crystal-lined cavities and See also:

VEINS

veins of granites, pegmatites and gneisses, for example, at Baveno and See also:

ELBA (Gr. AiWaXla; Lat. Ilva)

Elba in Italy, Alabashka near Mursinka in the Urals, Hirschberg in See also:

SILESIA

Silesia, Tanokami-See also:

YAMA (Sanskrit " twin," in allusion to his being twin with his sister Yami, traditionally the first human pair)

yama in the See also:

province Omi, See also:

JAPAN

Japan, and the Mourne Mountains in See also:

Ireland.' As a mineral of secondary origin orthoclase is sometimes found in cavities in basaltic rocks, and its occurrence in metalliferous mineral-veins has been observed. It has been formed artificially in the laboratory and is sometimes met with in See also:

FURNACE

furnace products. The commonest alteration product of orthoclase is See also:

KAOLIN

kaolin (q.v.); the frequent cloudiness or opacity of crystals is often due to partial alteration to kaolin. See also:

MICA

Mica and See also:

EPIDOTE

epidote also result by the alteration of orthoclase. (L. J.

End of Article: ORTHOCLASE

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