MINERALOGICAL AND ISOTOPIC EVIDENCE FOR PASSIVE CARBON SEQUESTRATION AT WOLLASTONITE MINES

The wollastonite (Wo) weathering reaction CaSiO₃ + CO₂ = CaCO₃ + SiO₂ is used as a model reaction for silicate weathering and carbonation, and has been the subject of extensive experimental investigation. Here we report the first characterization of this reaction in the field, occurring passively at Wo mines in the Adirondack region of New York. Crushed ore shows textural evidence for Wo dissolution and cementation by calcite (Cal) + silica (Si). Low δ¹³C values and filamentous biofilms suggest a microbial role in carbonation.

Cementation of crushed Wo is observed as hardened layers (≤30 cm) at the top of ore stockpiles, berms, and roadways at the Fox Knoll, Lewis, and Oak Hill mines in Essex County, and at the Valentine mine in Lewis County. These different mines exhibit varying extents of Cal mineralization and generally strong correlation in the abundances of texturally-late Cal and Si. Highly-cemented materials contain the most Cal at each site, ranging to a maximum of 2 wt% at Lewis, 6 wt% at Fox Knoll, and 20 wt% at Valentine. At Lewis and Fox Knoll, where primary Cal is essentially absent, less Cal and Si are present than is predicted from the amount of Wo dissolution observed, suggesting that aqueous Ca²⁺ and HCO^3- have left the system. At Valentine, where marble is common in the country rock, Cal, Si, and Wo are present in stoichiometric proportions in crushed ore. In 7/8 crushed ore samples δ¹³C(Cal) is distinct from marble δ¹³C, but it is possible that the addition of Ca²⁺ from dissolution of marble may have promoted Cal formation in cements at this site.

δ¹³C(Cal) in crushed ores also vary by deposit. At Fox Knoll δ¹³C averages -7.7±1.4‰ VPDB (n=13, lowest value -11.7‰), at Valentine δ¹³C averages 11.9±4.6‰ (n=8, lowest value -20.4‰), and at Lewis δ¹³C averages -17.8±5.5‰ (n=19, lowest value -29.5‰). We take these data as evidence for a (variable) microbial role at these sites. The extent of this microbial signature may be related to the timing of mining, with Fox Knoll (active 1953–1982) having the highest δ¹³C, Valentine (1977-present) having intermediate δ¹³C, and Lewis (1980–2016) having the lowest δ¹³C. It is possible that acid precipitation, which progressively increased beginning in the 1950s and peaked in the Adirondack region during the 1970s and 1980s, inhibited or changed the microbial role, particularly at Fox Knoll.