Wollastonite

Wollastonite is a calcium silicate (CaSiO₃), a chain-silicate pyroxenoid that typically forms where limestone is metamorphosed by heat. Wollastonite forms through contact metamorphism when siliceous limestone or dolomite is heated by an adjacent igneous intrusion. The key reaction occurs at temperatures above roughly 600°C: calcite (CaCO₃) reacts with quartz (SiO₂) to produce wollastonite (CaSiO₃) plus carbon dioxide gas. This decarbonation reaction, CaCO₃ + SiO₂ → CaSiO₃ + CO₂, is one of the classic reactions in metamorphic petrology and serves as a useful temperature indicator for geologists mapping contact aureoles.

The mineral typically develops in skarns, the distinctive calc-silicate rocks that form at the contact between carbonate rocks and intruding magma. At these contacts, hot magmatic fluids infiltrate the limestone, bringing in silica and other elements while driving off CO₂. The resulting rock assemblage often includes wollastonite alongside garnet, diopside, epidote, and vesuvianite. The width of the wollastonite zone depends on the size and temperature of the intrusion, ranging from centimeters to hundreds of meters.

Wollastonite crystals grow as elongated, acicular (needle-like) prisms that often form radiating sprays or fibrous masses. Two polymorphs exist: wollastonite-1A (triclinic, the common low-temperature form) and wollastonite-2M (monoclinic, forming above about 1,125°C). The industrial deposits mined today formed millions of years ago when large granitic plutons intruded limestone sequences, baking the surrounding rock into marble and skarn.

Wollastonite is characterized by its white to cream color, acicular crystal habit, and two perfect cleavages intersecting at approximately 84° and 96°. This cleavage angle is diagnostic and distinguishes it from most similar-looking minerals. The crystals often form fibrous or columnar aggregates with a splintery fracture. Hardness of 4.5-5 places it between fluorite and apatite.

Distinguish from tremolite (amphibole with 56°/124° cleavage angles, slightly higher hardness), from pectolite (very similar appearance but produces a needle-like pain if handled carelessly due to extremely fine fibers, has slightly lower hardness), and from sillimanite (higher hardness of 6-7, found in aluminous metamorphic rocks rather than calcareous ones). Wollastonite dissolves in hydrochloric acid without effervescence, unlike calcite which fizzes vigorously. The geological context of contact-metamorphosed limestone is a strong identification aid.