(_a_) ORTHORHOMBIC PYROXENES.—By extinction angle, the orthorhombic pyroxenes having always parallel or symmetrical extinction in sections parallel to _a_, _b_, or _c_, and by higher order interference colors. Also from hypersthene by absence of, or much fainter, pleochroism. Diallage and bronzite might be confused on account of pronounced pinacoidal parting, fibrous structure and inclusions; but may be distinguished by the presence or absence of extinction angles and also by the position of the optic axes relative to the best cleavage plates. (_b_) AMPHIBOLE.—See under amphibole. (_c_) EPIDOTE and CHRYSOLITE (Olivine). When light colored and granular, by examination in convergent light. The plane of the optic axes is parallel to the clino pinacoid (010), hence to the longitudinal axis and prismatic cleavage cracks, while in epidote it is at right angles to these directions and in chrysolite parallel to the base. Also yellow color is common in epidote but rare in pyroxene. REMARKS: Next to the feldspars pryoxene is the most common constituent of the igneous rocks. Diopside and fassaite (green) are found in contact rocks; also, what appear to be the same pyroxenes, in many eruptive rocks, as andesites, monzonites, etc. Malacolite (light green) is found in amphibolites and eclogites, where it may be associated with a greenish amphibole (smaragdite). Diallage (bladed and twinned) occurs in gabbros and pyroxenites. Common augite (brown) is found in the remaining basic eruptive rocks. In the schists the pyroxene is colorless. Finally augite occurs as a secondary product resulting from the “magmatic resorption” of hornblende and biotite. Chemical corrosion and mechanical deformation may occur. The green and brown augites when heated to redness on platinum foil may become red in color. In general the pyroxenes are not attacked by acids. H., 5 to 6. Sp. gr., 3.3 to 3.5. The sp. gr. of the pyroxenes is considerably higher than that of the amphiboles of similar composition, hence mechanical separations are possible. _Acmite_ (_Ægirine_) (Na pyroxenes).—Occur in green or brown, elongated prismatic crystals, often not very transparent and with marked pleochroism (like amphibole). Zonal coloring is common. When zonally intergrown with pyroxene the outer zone is ægirine. The elongation is ∥ a′ (distinction from amphibole whose elongation is ∥ c′). The index of refraction is higher than in the other pyroxenes (_n′_ = 1.792, α = 1.763, γ = 1.813) and the double refraction stronger (γ − α = 0.050). The _extinction_ angle is small (5°) and the optical character (—). The term _Ægirine-augite_ may be used to describe a soda, pleochroic augite with a large extinction angle.