_Twinning._—Very common, generally after _Carlsbad_ law, Figs. 18 and 69; the twinning boundary, dividing the section longitudinally, either being parallel to edges of crystal or bent or jagged. Twinning after _Baveno_ (twinning boundary diagonal, with the two parts extinguishing at the same time, but having a and c directions crossed in the two portions) and _Manebach_ laws less common.[98] [Illustration: FIG. 68.—Orthoclase, ortho pinacoid section showing cleavages intersecting at 90°, in augite-syenite. ] _Color._—Colorless or tinged by oxide of iron. Cloudy if decomposed. _Index of Refraction._—_n′_ = 1.523 (α = 1.519, γ = 1.526), hence no _relief_ and surface smooth. _Cleavage._—Varies and sometimes only seen in very thin sections, but is an important character and should always be searched for. It occurs perfect, parallel to base (001), and almost as perfect parallel to clino pinacoid (010). The two cleavages intersect at 90° in section parallel to the [_=b_] axis, Fig. 68. _Inclusions._—May be present and arranged in regular or zonal order, but not important. Do not occur in individuals of a second generation. ▄Polarized Light▄: _Pleochroism._—None. _▄Crossed Nicols▄_: _Double Refraction._—Very weak (γ − α = 0.007). _Interference Colors._—Lower first order, gray, white, etc., not quite so bright as colors of quartz and plagioclase. _Extinction._—Being monoclinic the extinction angle on base (001), with reference to clino pinacoid (010) cleavage cracks, is 0°. On clino pinacoid with reference to basal cleavage cracks, it is 5°. Some sections (notably in glassy sanidine grains) may appear dark during complete rotation. This is due to the fact that the axial angle is very small and the sections act approximately like those of a uniaxial mineral at right angles to the optic axis. _▄Convergent Light▄_:[99] Plane of optic axes in general at right angles to clino pinacoid (010) (plane of symmetry), Fig. 65, hence parallel to trace of basal cleavage; but in some sanidines parallel to plane of symmetry. Bx_{_a_} (a) Λ _a_ = 5° above. Axial angles vary, 2_E_ = 125° (orthoclase), 0°–50° (sanidine).[100] May appear uniaxial when axial angle is very small. Optical character (−). ▄Alteration▄: Very common to clay,[101] muscovite, hydrargillite, etc. Generally commences along the cleavage cracks, and when it has progressed very far the whole feldspar appears opaque or cloudy, and no perceptible change may take place between crossed nicols. As decomposition is very prevalent in many rocks, the orthoclase is rarely clear or pellucid. Epidote is often formed when accessory solutions are present. ▄Distinguished from▄: (_a_) The other FELDSPARS and MELILITE.—See under the latter minerals. (_b_) QUARTZ.—Feldspar is biaxial but when occurring in clear glassy grains (notably sanidine), which appear uniaxial in convergent light, may resemble quartz. When tested the optical character is (−), while that of quartz is (+).