A molecular dynamics study of Li speciation in hydrothermal fluids and silicate melts

Magmatic-hydrothermal ore-forming processes play an important role in the transport, enrichment and mineralization of Li. In this study, first principles molecular dynamics (FPMD) and classical molecular dynamics (CMD) techniques were employed to investigate the speciation of Li in silicate melts and hydrothermal fluids. FPMD results show that Li does not form stable ion pair with F or Cl in silicate melts, but forms four-fold Li-O coordination. Association constants of Li-F/Cl ion pairs in hydrothermal fluids under a wide range of T-P conditions (i.e., temperatures up to 1273 K and fluid densities ranging from 0.1 g/cm(3) to 1.0 g/cm(3)) derived with constrained CMD show increasing stability with temperature increasing or fluid density decreasing. Together with previous studies on Na/K-F/Cl ion pairs, an association constant database is provided for alkali halides ion pairs. It is found that Li-F ion pair is dominant in geological vapors.