Hurlbut, Cornelius S., Jr. Department of Geological Sciences, Harvard University, Cambridge, Massachusetts.
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A mineral inosilicate with composition CaSiO3. It crystallizes in the triclinic system in tabular crystals (see illustration). More commonly it is massive, or in cleavable to fibrous aggregates. There are two good cleavages parallel to the front and basal pinacoids yielding elongated cleavage fragments. Hardness is 5–5½ on Mohs scale; specific gravity is 2.85. On the cleavages the luster is pearly or silky; the color is white to gray. Wollastonite is the most common of three polymorphic forms of CaSiO3, the other two being pseudowollastonite and parawollastonite. Pseudowollastonite, a high-temperature triclinic form, is very uncommon in rocks but may be a constituent of synthetic CaO-SiO2 systems and of slags and glasses. Parawollastonite, a monoclinic form, is only rarely found in Ca-rich rocks. Wollastonite, by far the most common polymorph, occurs abundantly in impure limestones that have undergone contact metamorphism. Resulting assemblages may consist of calcite-diopside-wollastonite with variable amounts of tremolite, clinozoisite, and grossularite. Wollastonite occurs sporadically in regionally metamorphosed calcareous sediments as well. It is found in large masses in the Black Forest of Germany; Brittany, France; Chiapas, Mexico; and Willsboro, New York, where it is mined as a ceramic material. See also: Silicate minerals
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