Description:
<jats:title>Abstract</jats:title><jats:p>Vaterite, a CaCO<jats:sub>3</jats:sub> polymorph, is a rare mineral that is said to be metastable under all known conditions. According to the literature, vaterite precipitated from carbonate solution recrystallizes spontaneously to calcite or aragonite. Yet vaterite has been identified in hard tissues of organisms, in gallstones, in contact metamorphic aureoles, in zones of thermal metamorphism, in a meteorite, and in cone-in-cone concretions. Newly precipitated vaterite has formed at the expense of carbonate rock in drilling fluids in wells of New York, Michigan, Nevada, Texas, and New Zealand. Petrographic examination reveals a light brown core of Ca<jats:sub>3</jats:sub>SiO<jats:sub>5</jats:sub> surrounded by a colourless rim of vaterite. The δ<jats:sup>18</jats:sup>O<jats:sub>PDB</jats:sub> of New York vaterite is −12.4‰ and that of the Michigan vaterite is −17.6‰, which reflect the oxygen isotopic composition of meteoric freshwater used in drilling. The δ<jats:sup>13</jats:sup>C<jats:sub>PDB</jats:sub> value of −19.2‰ for New York vaterite and that of −17.6‰ for Michigan vaterite suggest that natural gas dissolved original carbonate in the subsurface. Drilling records from both wells indicate that natural gas was released into the drilling muds from the formations exposed at the time vaterite was encountered. Crossplots of the oxygen and carbon isotopic ratios overlap those of spurrite rocks in thermal metamorphic zones in Israel. A C-14 radiocarbon analysis of the Michigan vaterite gave an age of 953±39 yr. BP. 88.8±0.6% is modern carbon and 11.2% is dead carbon. Hence this carbon, and therefore the vaterite, is essentially modern. A sample of the New York vaterite yielded a modern age.</jats:p>