An important group of inosilicates (chain silicates) comprising the orthorhombic pyroxenes (orthopyroxenes) and the monoclinic pyroxenes (clinopyroxenes) with the general formula XYZ₂O₆, where X = Mg, Fe, Ca, or Na; Y = Mg, Fe, Fe³⁺, or Al; and Z = Si (and some Al substitution). The main orthopyroxenes are enstatite and orthoferrosilite; the main clinopyroxenes include diopside, hedenbergite, augite, pigeonite, and jadeite, and also the alkali pyroxenes aegirine and aegirine augite (note that wollastonite, although similar to the pyroxenes, has a different atomic structure); sp. gr. 3.0–3.5; hardness 5–6; colours variable, but usually dark greens, browns, or black; vitreous lustre; crystals usually short or columnar prisms; well-marked cleavage; widely distributed in both igneous and metamorphic rocks.