EVIDENCE FOR LIQUID-LIQUID SEPARATION AT THE ONSET OF CACO3 MINERALIZATION: IMPLICATIONS FOR CARBONATE DEPOSITION IN NATURAL ENVIRONMENTS

Compositional signatures within authigenic and biogenic carbonate phases are often used as indicators of past environmental conditions. Such efforts are underpinned by assumptions about the microscopic details of mineral formation. In the classical sense, carbonate minerals are presumed to nucleate directly from solution by overcoming a size-dependent free energy barrier that scales as the ratio of the macroscopic mineral-water interfacial tension cubed to the square of the thermodynamic supersaturation. However, experimental observations of the early stages of carbonate mineralization demonstrate that under certain conditions the formation of crystalline carbonates is preceded by the apparently spontaneous appearance of nanoscopic clusters that aggregate to produce metastable amorphous phases. Further, there is evidence to suggest that the level of magnesium incorporation into calcite formed via such intermediates may also be enhanced. Therefore, development of environmental proxies based on non-empirical relationships requires an understanding of the molecular level processes driving carbonate mineralization and how they vary in response to local environmental parameters.This research (Wallace et al., in press, Science) uses molecular dynamics techniques to probe the initial formation of hydrated calcium carbonate cluster species and lattice gas simulations to explore the general behavior of clusters at the onset of mineralization. The results suggest the growth of carbonate clusters may indeed proceed in the absence of any significant thermodynamic barrier. Moreover, the dynamical properties of the clusters are consistent with that of a dense liquid phase. Coalescence and dehydration of the nanoscale droplets results in the formation of a phase whose structure is consistent with that of amorphous calcium carbonate. These findings indicate that a spontaneous liquid-liquid phase separation may occur within the range of supersaturations spanned by natural waters.