PLAGIOCLASE, . an important group of rock-forming minerals, constituting an isomorphous series between albite, or soda-felspar and anorthite, or lime-felspar. Intermediate members are thus soda-lime-felspars, which in their crystallographical, optical and other physical characters vary progressively with the chemical composition between the two extremes albite (NaAlSisOg) and anorthite (CaAUSijOg). This variation is continuous in the series, but specific names are applied to members falling between certain arbitrary limits, viz.: Albite, Ab ( = NaAlSisOg) ; Oligoclase, Ab 6 Anj to Ab 3 Ani; Andesine, Ab 3 Ani to AbiAni; Labradorite, AbiAni to AbiAnj; Bytownite, AbiAnj to AbiAne; Anorthite, An (=CaAl 2 Si20s). All the members of the series crystallize in the anorthic (triclinic) system. They possess a perfect cleavage parallel to the basal pinacoid P (ooi) and a somewhat less pronounced cleavage parallel to the pinacoid M (oio). The angle between these two cleavages varies from 86 24' in albite to 85 50' in anorthite. It was on account of the oblique angle between the cleavages that A. Breithaupt in 1847 gave the name plagioclase (Gr. ir\6.yios, oblique, and K\av, to cleave) to these felspars, to distinguish them from the orthoclase felspar in which the corresponding cleavage angle is a right angle. It should be noted that the potash and potash-soda-felspars, microcline (q.v.) an anorthoclase, though also anorthic, are not included in the plagioclase series of soda-lime-felspars. Crystals are usually tabular in habit, parallel to the plane M, as shown in the accompanying figure; sometimes, however, they are flattened parallel to P, this being a characteristic habit of the pericline variety of albite; microlitic crystals forming the ground-mass of volcanic rocks are usually elongated in the direction of the edge between P and M.
Twinning is an important character, which is almost invariably present and affords a ready means of distinguishing the plagioclases from other felspars. Most frequent is the twinning according to the " albite law " with M as twin-plane. One half of the twin is turned through 1 80 about the normal to this plane and the two portions are united along the same plane (for figures of twinned crystals see ALBITE). The basal planes of the two portions are inclined to each other at a salient or re-entrant angle of 7 12' in albite and 8 20' in anorthite. This twinning is usually polysynthetic, being many times repeated, and giving rise to numerous thin lamellae, which are the cause of the fine striations on the cleavage planes P and parallel to the edge PM , so characteristic of the plagioclases as seen in hand specimens. Viewed in polarized light, thin sections of twinned crystals show a very characteristic banded structure parallel to M. A second twinlaw is known as the "pericline-law" because of its frequent occurrence in pericline. Here the axis of rotation is the edge x P (the crystallographic axis 6) and the plane of composition is the " rhombic section " : the latter is a plane which intersects the prism faces T and / in a rhomb; it is not a possible face of the crystal, and its position varies in the different species. In addition to being twinned according to these two laws, plagioclase may also be twinned on the Carlsbad-, Baveno- and Manebach-laws, as in orthoclase (q.v.).
The optical characters of the plagioclases have been the subject of much study, since they are of great value in determining the constituents of rocks in thin sections under the microscope. The mean indices of refraction and the angles of extinction on the cleavages P and M are given in the accompanying table. (The meaning of the + and directions will be seen from the figure, where the face P slopes from left to right, i.e. the angle between the normals to the faces lettered P and Afis less than 90). The extinction angles on other faces, or in sections of known orientation in the crystal, also give constants of determinative value: for example, in sections perpendicular to the plane M the extinctions, which in crystals twinned according to the albite-law are symmetrical with respect to this place, reach the maximum values given in the table. Not only do the directions of extinction (axes of light-elasticity) vary in the different species, but also the optic axial angle, so that while albite is optically positive, anorthite is negative, and a member near andesine has an axial angle of 90. The figures seen in convergent polarized light through the P and M cleavages are characteristic of the different species. A detailed summary of the optical characters and their employment in discriminating the several members of the plagioclase series is given by H. Rosenbuch, Mikroskopische Physiographie der Mineralien und Gesteine (4th ed. Stuttgart, 1905)- The plagioclases occur as primary constituents of igneous rocks of almost every kind, and are also frequent as secondary minerals in metamorphic rocks. Albite and oligoclase are more characteristic of acidic rocks, whilst the basic members at the anorthite end of the series are characteristic of rocks containing less silica. The composition may, however, vary even in the same crystal, zoned crystals with a basic nucleus and with shells successively more and more acid towards the exterior being common.
For further particulars respecting individual species and their modes of occurrence see ALBITE ; ANDESINE ; ANORTHITE ; B YTOWNITE LABRADO RITE; OLIGOCLASE. (L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)