_▄Convergent Light▄_: All plagioclases show the emergence of a bisectrix,[107] more or less oblique, on brachy pinacoid (010) cleavage faces. These cleavage faces show no twin lamellæ, unless twinning after _Pericline_ law occurs, in which case the determination is much more complicated. The axial angle is large, 2_E_ = 155° (Albite). Optical character, depending on variety, (+) or (−). ▄Alteration▄: Partly the same as in orthoclase, forming clay, muscovite, etc. Calcite and epidote are more common as side-products, and zeolitization also occurs in some rocks. The plagioclases decompose more easily than orthoclase. ▄Distinguished from▄: (_a_) ORTHOCLASE.—By repeated twinning after _Albite_ law, giving between crossed nicols a series of alternate dark and light bands. When _Albite_ twinning is absent the distinction is very difficult. (_b_) MICROCLINE.—By common absence of the microcline “_gridiron_” structure between crossed nicols. Methods for Optical Determination of the Plagioclases.[108] The correct determination of the particular plagioclase is of the greatest importance in the classification of rocks, and it is no longer sufficient to simply determine the feldspar as either orthoclase or plagioclase. A quantitative analysis of isolated material would lead most surely to the desired result, but has many objections. Modern optical methods now permit of a very accurate and convenient determination under ordinary circumstances. But of course these methods involve a knowledge of the approximate orientation of the section tested. When this section is not a definite cleavage fragment, its orientation can best be determined by convergent light tests. Only an outline of these methods can be here given, and reference should be made to more complete works for an elaborate discussion of the subject. It is very convenient to have at hand a set of glass models of the plagioclases, showing location of plane of optic axes, vibration directions and crystal axes.[109] (1) _Schuster’s_ method of recognizing the different feldspars by extinction angles measured on the cleavage plates[110] is very precise, but not always applicable for crystals in rock sections. EXTINCTION ANGLES: EXTINCTION ANGLES: ON BASE, MEASURED FROM TRACE OF ON BRACHY PINACOID, MEASURED FROM PINACOIDAL CLEAVAGE. TRACE OF BASAL CLEAVAGE. Albite + 4° Albite +19½° Oligoclase, Ab_{4}An_{1} + 2° Oligoclase, Ab_{4}An_{1} + 8° Labradorite, Ab_{1}An_{1} − 5½° Labradorite, Ab_{1}An_{1} −20° Anorthite −36½° Anorthite −41½° Confusion may here arise between albite and labradorite if disregard be had to signs, but the more acid oligoclase is readily distinguished from the basic anorthite.