Wollastonite is a common mineral in regionally metamorphosed medium to high-grade calc-silicate rocks and carbonate-bearing contact metamorphic rocks (e.g., tactites).  It is colourless and has moderately high positive relief relative to feldspars (e.g., compared to plagioclase in A).  It occurs as blades, fibres or prisms and has cleavage that resembles that of the pyroxenes (A), though the orientation is different.  Maximum interference colour is first order yellow and twins on {100} are common (B).  Extinction is typically parallel to crystal length, or nearly so. Sample is from the Valley Springs Gneiss, Llano, Texas.  Fields of view are 2.2 mm across. A ppl, B x-nicols.