ORTHOCLASE  , an important

rock-forming mineral belonging to the felspar group (see FELSPAR) . It is a potash-felspar, KAlSi303, and crystallizes in the monoclinic system . Large and distinctly developed crystals are frequently found in the drusy cavities of granites and pegmatites . Crystals differ somewhat in 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 tabular (fig . 2), being flattened parallel to the clino-pinacoid or plane of symmetry b (oro), as in the variety sanidine (aavis, vavlbos, a 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 granite quarries at Baveno in Italy . Twinning is frequent, and there are three well-defined twin-laws: (1) Carlsbad twins (fig . 4) . Here the two individuals of the twin interpenetrate or are united parallel to the clino- pinacoid: one individual may be brought into the position of the other by a rotation of 18o° about the vertical crystallographic axis or 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

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 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 angle between these two cleavages is 900, hence the name Catholic Apostolic Eastern Church "), the 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 Monte Somma, Vesuvius .

The

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 change from one position to the other . In all cases, however, the acute negative 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 rich in soda, at 10-12° . The mean refractive index is 1.524, and the double refraction is weak (o•oo6) . Analyses of orthoclase usually prove the presence of small amounts of soda and lime in addition to potash . These constituents are, however, probably present as plagioclase (albite and 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 aventurine-felspar, known as perthite, from Perth in Lanark county, Ontario . Frequently, however, as in microperthite and cryptoperthite, this is on a microscopic scale or so minute as to be no longer recognizable . These directions (Too)•and (8or) are planes of parting in orthoclase, and along them alteration frequently takes place, giving rise to schiller effects . Moon-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 augite-syenite of Fredriksvarn and Laurvik in southern Norway, which is much used as an ornamental stone . The same effect is shown to a lesser degree by murchisonite, named in honour of Sir R.I . Murchison, from the Triassic conglomerate of Heavitree near Exeter . Orthoclase forms an essential constituent of many acidic igneous rocks (granite, syenite, porphyry, trachyte, phonolite, &c.) and of crystalline schists and gneisses .

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

Westmorland, Cornwall, &c.) it occurs as em-bedded crystals with well-defined outlines, but usually it presents no crystalline form . In the trachyte of the Drachenfels and the Laacher See in Rhenish Prussia there are large porphyritic crystals of glassy sanidine . The best crystals are those found in the crystal-lined cavities and veins of granites, pegmatites and gneisses, for example, at Baveno and Elba in Italy, Alabashka near Mursinka in the Urals, Hirschberg in Silesia, Tanokami-yama in the province Omi, Japan, and the Mourne Mountains in 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 furnace products . The commonest alteration product of orthoclase is kaolin (q.v.); the frequent cloudiness or opacity of crystals is often due to partial alteration to kaolin . Mica and epidote also result by the alteration of orthoclase . (L . J .