_▄Convergent Light▄_: Axial plane ∥ (010), _i. e._, at right angles to the elongation of crystal and cleavage cracks, Fig. 61. Bx_{_a_.}-(a) Λ _ć_= 3° behind. Basal cleavage flakes show the almost ⟂ emergence of an optic axis. Axial angles very large (2_E_ > 180°). Optical character (−). ▄Alteration▄: Does not take place readily. ▄Distinguished from▄: Light colored MONOCLINIC PYROXENE.—By having plane of optic axes at right angles to cleavage cracks and direction of elongation; while in pyroxene plane of optic axes is parallel to parallel prismatic cleavage cracks or bisects the angle between intersecting cracks. Furthermore the yellow color is rare in pyroxene. REMARKS: Epidote is essentially a secondary mineral, resulting from the alteration of the feldspars and the ferro-magnesium silicates. It is found in crystalline schists (especially those containing hornblende), gneiss, gabbro, diorite, diabase, lime-silicate hornstones, contact rocks, etc. Epidote is partially decomposed by hydrochloric acid. The Fe rich epidote can be changed to an intense color by “glowing” in the air. H., 6 to 7. Sp. gr., 3.32 to 3.45. _Piedmontite_ (containing Mn).—Red in color. Very pleochroic, red to yellow. Found in crystalline schists, the porphyrite of Scotland, the famous “Porfido rosso antico” of Egypt and in certain Japanese mica schists. ZOISITE. Essentially orthorhombic? members of Epidote group.[95] COMPOSITION: Like epidote but without any Fe. ▄Usual Appearance in Sections▄: Similar to epidote or in columnar aggregates. Often intergrown with epidote. Distinguished from epidote by general absence of color (colorless to yellowish) and pleochroism; by slightly lower refractive index (_n′_ = 1.699 to 1.720) and by much weaker double refraction (γ − α = 0.005 and less). The interference colors are very low order, gray to white, but anomalous colors are often seen (yellow or prussian blue). Extinction is in general parallel to pinacoidal cleavage cracks (except in clinozoisite). The plane of the optic axes may be parallel or at right angles to cleavage cracks, and the optical character is (+). REMARKS: Generally a secondary mineral. Found in crystalline schists, amphibolites, contact rocks, eclogite, etc., and in “saussurite.” May be hard to distinguish from vesuvianite and apatite. ALLANITE, Orthite. Monoclinic member of Epidote group. COMPOSITION: Like epidote but containing cerium.