WOLLASTONITE, a rock-forming mineral, consisting of calcium metasilicate, CaSiO3, crystallizing in the monoclinic system and belonging to the pyroxene (q.v.) group. It differs, however, from other members of this group in having cleavages, not parallel to the prism-faces, but in two directions perpendicular to the plane of symmetry. Crystals are usually elongated parallel to the axis of symmetry and flattened parallel to the ortho-pinacoid, hence the early name "tabular spar"; the name wollastonite is after W. H. Wollastone. The mineral usually occurs in white cleavage masses. The hardness is 5, and the specific gravity 2.85. It is a characteristic product of contact-metamorphism, occurring especially, with garnet, diopside, &c., in crystalline limestones. Crystals are found in the cavities of the ejected limestone blocks of Monte Somma, Vesuvius. At Santa Fé in the state of Chiapas, Mexico, a large rock-mass of wollastonite carries ores of gold and copper; here are found large pink crystals which are often partially or wholly altered to opal. (L. J. S.)