Experimental precipitation of cryogenic calcite and vaterite

Cryogenic cave carbonates (CCCs) are speleothems precipitated from freezing water bodies in caves. A series of scholarly papers have explored natural CCC precipitation kinetics, crystal habit, mineralogy, crystallography and geochemistry. However, cave depositional environments are complex multi-parameter systems, and the inherent difficulty of unravelling the physicochemical parameters of ancient speleothem formation also applies to CCCs. In order to test previous models and concepts, experiments were conducted by cooling calcium-rich bicarbonate water under controlled laboratory conditions. The experiments involved calcite precipitation at different durations (6 to 91 days) and temperatures (1, +0, -0.5, -0.7, -1 and -2 °C). The results show that subtle changes in temperature and calcite saturation index (SIcc) remarkably affect the CCC morphological variability and mineralogy. Rhombohedral calcite crystals precipitated both in experiments with and without ice. In contrast, spherulitic crystals (vaterite) precipitated from near-completely frozen waters with high calcite saturation indices. Experimental water monitoring revealed increased calcite precipitation followed by decreased water conductivity values. The formation of CCCs is accompanied by non-equilibrium reaction kinetics, which tends to enrich the isotopic composition of the parental water. Longer experimental durations lead to increasing CO₂ degassing and ¹³C enrichment of the water. Data from the first experimental work dealing with cryogenic carbonate precipitation and isotope geochemistry, as documented here, shed light on the origin of these peculiar speleothems and provides constraints to interpret morphologies and isotopic compositions of ancient CCCs.