PLAGIOCLASE, an important group of rock-forming min erals constituting an isomorphous series between albite felspar and anorthite felspar. The intermediate members are soda-lime felspar which in their crystallographical, optical and other physi cal properties vary progressively between the two extremes, albite (Ab) and anorthite (An) The name plagioclase is derived from the Gr. rXecycos, "oblique," and ativ, "to break," in allusion to the oblique angle between the cleavages. Names are applied to members of the plagioclase group falling between certain arbitrary composition limits, those now commonly adopted being albite oligoclase andesine labradorite (Ab.-Ab.), bytownite (Abso-Abi.), anorthite (Abic--An..). The pure end members are practically unknown in nature. All the plagioclases crystallize in the triclinic system, and as members of an isomorphous series show closely similar crystallographic constants. They possess a perfect cleavage parallel to the basal pinacoid (001) and a less pronounced cleavage parallel to the pinacoid (cm), the angle be tween the two cleavages varying progressively from 86° 24' in albite to 85° 5o' in anorthite. The molecular volumes are closely similar (Ab ioo.i) (An Is:N.5). The habit of well-formed crystals is usually tabular on the plane oio, sometimes flattened parallel to ooi or, as in microlitic plagioclases of volcanic rocks, elongated in the direction of the edge ooi :010. Twinning is a very important character of the plagioclases, being almost invariably present and affording a ready means of distinguishing them from other fel spars. The chief twin laws are the Carlsbad, albite and pericline ; the latter two, being commonly polysynthetic, give rise to numer ous thin lamellae, which are the cause of the fine striations seen on cleavage planes, and of the banded character visible in thin sections of the minerals examined in polarized light.
With the exception of the sodic end member (albite), which, owing to the high viscosity of its melts, is with difficulty crystal lized, the plagioclases are readily prepared from dry melts of the component oxides. They show a continuous melting curve, the
intermediate members exhibiting a melting interval. The temper atures of beginning of melting (solidus) have been determined in the laboratory for compositions ranging from pure An to and of completion of melting (liquidus) for a range An to Ani. These data are shown below.
The density and mean refractive index of the plagioclase felspars vary progressively, the extremes being albite, D = 2.6o5, anorthite, D = 2.765, µ=1.587.
Since the plagioclases are of great importance in petrology much detailed study of their optical properties has been under taken, so that it is now possible to determine by simple examina tion of thin slices of these minerals the exact chemical composi tion and crystallographic orientation of the selected section. For this purpose, the chief determinations are the refractive indices, and the optical extinctions of oriented sections, principally on cleavage flakes (ooi and oio), sections cut perpendicular to the optic axes and bisectrices, and especially those perpendicular to the plane of albite twinning (oio) in multiple-twinned crystals. Other optical determinations are available for discriminating the various members of the group.
BIBLIOGRAPHY.—FOr a detailed summary of the optical characters, and their employment in the recognition of the quantitative composi tion of the felspars, the following should be consulted: H. Rosen busch, Mikroskopische Physiographie der Mineralien und Gesteine (Band i., Zweite Hiilfte, 5 Auflage, revised by 0. Mugge, 1927) ; L. Duparc and M. Reinhard, "La Determination des Plagioclases" (Mem. Soc. Phys. Hist. Nat. Geneve, xl., 1924, fasc. I.) ; A. N. Winchell, Elements of Optical Mineralogy (pt. 2, 1927, pp. 277-341). For a recent preliminary X-ray study of the plagioclase felspars, reference may also be made to E. Schiebold, Fortschritte der Mineralogie, Kristallographie und Petrographie (Band xii., 1927, pp. 78-82). A more detailed account of the occurrence of the various members is given under ALBITE, OLIGOCLASE, ANDESINE, LABRADORITE,