_Color._—From almost colorless through green (diopsides, Na pyroxenes, etc.) to brown (augites); the red to brownish-red color of certain augites has been considered due to manganese. Yellow color very rare. _Index of Refraction._—_n′_ = 1.68 to 1.72 (α = 1.671 to 1.706, γ = 1.700 to 1.728), hence _relief_ high and surface rough. _Cleavage._—More or less perfect parallel to prism of 87° 06′. Cleavage cracks distinct and numerous, but not generally running uninterruptedly through crystal, Figs. 12 and 52. Cleavage not so perfect as that of amphibole. _Parting._—Diallage and diopside have distinct parting parallel to ortho pinacoid (100), Fig. 53. Some crystals may show parting parallel to base (001). _Inclusions._—Tabular microscopic interpositions, similar to those in bronzite, may occur in diallage. The iron ores, apatite, etc., may occur in augite. ▄Polarized Light▄: [Illustration: FIG. 53.—Diallage, cross-section. ] _Pleochroism._—Usually not noticed, and in general only appearing as different shades of the same color. In some cases (diallage, fassaite and Na rich augite) well marked, a and c green to yellowish green and b brownish to reddish-brown; hence pleochroism not intense in sections showing extinction angles. When Ti is present, violet parallel to _b_. ▄_Crossed Nicols_▄: _Double Refraction._—Strong (γ − α = 0.022 to 0.029), being stronger in the pale or colorless pyroxenes. _Interference Colors._—Second order, hence always bright tints. _Extinction._—Symmetrical in sections (through _b_ axis) showing intersecting cleavage lines, in such cases bisecting the angles of the cleavage. In sections showing parallel cleavage lines, only parallel in ortho pinacoid (100) sections, in all other sections an extinction angle being observed. The maximum extinction angle is large, lies in the obtuse angle, varies with the chemical composition from 36° 30′ to 54°, and is only obtained when the section of the crystal is parallel to the clino pinacoid (010), Fig. 54, varying from this angle to 0°, when the section is parallel to the ortho pinacoid (100). In Ti and Na pyroxenes the inclined dispersion is so great that extinctions are not sharp, but instead a change takes place in the interference color from bluish to brownish. ▄_Convergent Light_▄: Axial plane parallel to clino pinacoid (010). Fig. 54. A cleavage flake parallel to ortho pinacoid (100) shows the emergence of an optic axis (orthorhombic pyroxene parallel to best pinacoidal cleavage would not show figure). Bx_{_a_}.(c) Λ _ć_ = 36° to 54° front. Axial angles large (2_E_ = 70° to 112°). Optical character (+). The interference figures are distinct on account of the strong double refraction. ▄Alteration▄: May take place to chlorite, serpentine or amphibole (uralitization[92]), depending on the chemical composition and the conditions producing the change. [Illustration: FIG. 54.—Diopside, clino pinacoid section. ] ▄Distinguished from▄: