# 1911 Encyclopædia Britannica/Orthoclase

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​ORTHOCLASE, an important rock-forming mineral belonging to the felspar group (see Felspar). It is a potash-felspar, KAlSi$_3$O$_8$, 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. 1), 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 (010), as in the variety sanidine (σᾰνίς, σᾰνίδος, a board); or again the crystals may be elongated in the direction of the edge between 𝑏 and the basal plane 𝑐 (001), 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 180° 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 (001); examples of this rarer twin were first found at Manebach in Thuringia.
| Fig. 1. | Fig. 2. | Fig. 3. |
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 90°, hence the name orthoclase (from the Gr. ὀρθος, right, and κλᾶν, 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 (110).
The hardness is 6, and the sp. gr. 2·56. Crystals are sometimes colourless and transparent with a glassy aspect, as in the varieties adularia, sanidine and the rhyacolite of Monte Somma, Vesuvius.
| Fig. 4. | Fig. 5. | Fig. 6. |
| Twinned Crystals of Orthoclase. |||||
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 (0·006).
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 (100) or the pinacoid (801), 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 (100) and (801) 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 Fredriksvärn 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 embedded 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 n 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. S.)