Wollastonite (CaSiO3) is a silicate mineral that occurs in metamorphosed carbonate rocks.
Wollastonite belongs to a group of chain silicates named pyroxenoids. These are minerals that are similar to pyroxenes but their crystal chains are distorted (not straight). Compared to pyroxenes, these minerals are rare. There are only three common minerals among pyroxenoids (wollastonite, rhodonite, and pectolite) but even these are found in few rock types that are not very voluminous. Wollastonite as a most important of these minerals occurs only in a handful of places where it is abundant enough to make mining worthwhile. One of them is Willsboro in New York State (two photos below).
Wollastonite forms when limestone reacts with silicate fluids:
CaCO3 (calcite) + SiO2 (quartz) → CaSiO3 (wollastonite) + CO2 (carbon dioxide)
Wollastonite may form when impure (contains silica) limestone (or dolostone) gets buried deep enough for the necessary metamorphic reactions to take place (regional metamorphism) or when magmatic fluids intrude the limestone body (metasomatism which produces skarns). In either case many other minerals may form as well. Diopside, calcite, dolomite, tremolite, andradite, grossular, plagioclase, epidote, vesuvianite, etc. may be associated with wollastonite. It may be very rarely found in some igneous rocks.
Pure wollastonite is white. It is usually relatively pure and therefore white but gray and light green colors are common also. It is typically fibrous, columnar, or bladed. It may be very similar to tremolite (amphibole group mineral). Unfortunately, these two love to occur together which complicates the identification process. Tremolite is light green (but usually darker) in color and forms columnar or acicular crystals.
Wollastonite with diopside (green) from a mine in Willsboro, New York State, USA. The width of the rock sample is 8 cm.
Calc-silicate minerals diopside (green), andradite (brown), and wollastonite (white) in a skarn from a mine in Willsboro, New York State, USA. The width of the view is 5 cm.
Sample from Lahore, Pakistan. The width of the specimen is 8 cm.
Wollastonite, tremolite (green), and actinolite (black) from Bastnäs, Sweden. The width of the specimen is 9 cm.