Oligoclase, _n_(NaAlSi_{3}O_{8}) + CaAl_{2}Si_{2}O_{8}, or _n_ Ab + An, _n_ = 2 to 6. Labradorite, NaAlSi_{3}O_{8} + _n_(CaAl_{2}Si_{2}O_{8}), or Ab + _n_ An, _n_ = 1, 2 or 3. Anorthite, CaAl_{2}Si_{2}O_{8}. ▄Usual Appearance in Sections▄: Much the same as orthoclase. Lath-shaped[106] forms and microlites very common, especially in the acid series. [Illustration: FIG. 70.—Plagioclase, showing narrow lamellæ, in diabase. (From Cohen.) ] [Illustration: FIG. 71.—Plagioclase, showing broad lamellæ, in gabbro. (From Cohen.) ] _Twinning._—Polysynthetic, after _Albite_ law, almost universal; the twinning appearing between crossed nicols as a series of dark and light bands, bounded by parallel edges, Figs. 70 and 71. The twin lamellæ are parallel to brachy pinacoid (010), hence not observed in sections parallel to this pinacoid. The lamellæ may appear irregular and interrupted, and seem to be broader in the basic than in the acid series. When this twinning fails, however, as in the basic plagioclases in certain metamorphic rocks, the determination becomes very difficult. In some cases polysynthetic twinning, after both _Albite_ and _Pericline_ laws, may take place at the same time, giving rise to a structure somewhat similar to that of microcline, Fig. 72. In addition the polysynthetic crystals may be twinned like orthoclase after _Carlsbad_ and _Baveno_ laws. The general characters are the same as in orthoclase with the following differences: _Indices of Refraction_: _n′_ = 1.535 (α = 1.532, γ = 1.540) Albite. _n′_ = 1.541 (α = 1.537, γ = 1.545) Oligoclase, Ab_{4}An_{1}. _n′_ = 1.559 (α = 1.555. γ = 1.563) Labradorite, Ab_{1}An_{1}. _n′_ = 1.582 (α = 1.575, γ = 1.588) Anorthite. The surface of anorthite appears slightly rougher than that of orthoclase. _Cleavages_, parallel to base (001) and brachy pinacoid (010), never intersect at right angles, as is the case in sections of orthoclase parallel to [_=b_] axis. This is due to the triclinic system of crystallization, but the divergence from a right angle is small (93° 36′ to 94° 10′). [Illustration: FIG. 72.—Plagioclase, showing crossed lamellæ, in olivine-gabbro. (From Cohen.) ] _Inclusions_ at times may be quite important, as the vitreous inclusions of oligoclase in andesites, etc., and the iron ore inclusions and other microlites in labradorite. The arrangement of these inclusions may be zonal or in parallel orientation. _Double refraction_ is a little stronger than for orthoclase (γ − α = 0.008 to 0.013 (anorthite)), hence producing slightly brighter interference colors in sections of the same thickness. _Extinction_ takes place in all sections unsymmetrically with respect to crystallographic, twinning or cleavage lines (as these minerals are triclinic); hence extinction angles are always observed.