enstatite (En) An important orthorhombic pyroxene (orthopyroxene, or opx) Mg₂Si₂O₆ and end-member of an isomorphous series, the other end-member being orthoferrosilite (Fs) Fe₂Si₂O₆; this series forms the base of a compositional equilateral triangle whose third apex is wollastonite (CaSiO₃) and into which the common pyroxene minerals (augite, diopside, pigeonite) can be plotted, as well as olivine; sp. gr. 3.2 (En) to 3.96 (Fs); hardness 5.5; colourless or greyish-white, with a green tinge, or brownish-green; vitreous lustre; crystals are prismatic or tabular, but normally forming very irregular grains; cleavage good prismatic {110}, poor {010}, {100}; occurs in magnesium-rich magmatic rocks (e.g. peridotites, gabbros, norites, and basalts) and in contact metasomatic zones associated with these rocks; and in partial melt envelopes around very high-temperature basic igneous intrusions.

wollastonite Member of the nesosilicates (chain silicates) CaSiO₃ and associated with the pyroxenes, although it does not possess a pyroxene atomic lattice and is termed a pyroxenoid (along with pectolite (Ca₂NaH[SiO₃]₃) and rhodonite); sp. gr. 2.8–3.1; hardness 4.5–5.0; triclinic; white to grey; vitreous to pearly lustre; crystals tabular, prismatic, also occurs massive and cleavable, or fibrous; cleavage perfect {100}; occurs in metamorphosed, silicaceous limestones, and alkaline, igneous rocks, and associated with calcite, epidote, and tremolite; soluble in hydrochloric acid with the separation of silica. It is used as a source mineral for rock wool of high strength and with long fibres, and is named after the British mineralogist W. H. Wollaston.