THERMODYNAMICS OF BORAX-TINCALCONITE EQUILIBRIUM

The stability of borate minerals is an important consideration for assessing biogeochemical processes in primordial and extant Earth-surface environments. Despite the geological and industrial importance of borate minerals, their stability remains poorly constrained. In the present study, the thermodynamic properties of dehydration of borax [Na2B4O 5(OH) 4•8H2O] to tincalconite [Na6[B4O5(OH) 4]3•8H2O] were measured by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hydrofluoric (HF) acid solution calorimetry, X-ray powder diffraction (XRPD), and equilibrium observations. Observations of equilibrium between borax and tincalconite were conducted as a function of temperature and relative humidity (RH) using saturated salt solutions. Mass changes in TGA and isotherm measurements confirm the stoichiometry of tincalconite to be Na6[B4O5(OH) 4]3•8H2O. Relative humidity in equilibrium with borax and tincalconite increases from 59%±6% at 25ºC to 73% ±2% at 40ºC to 88%±8% at 55ºC and 65ºC. At 65ºC and 95% humidity, borax deliquesced. The increase in the equilibrium constant for the dehydration reaction is consistent with an enthalpy of dehydration (to water vapor) of 55.3 ± 2.0 kJ/mol of H2O. This is also consistent with the enthalpy of dehydration determined by HF solution calorimetry of 54.33 ± 0.24 kJ/mol of H2O and by DSC of 55.6kJ/±1.1 kJ/mol of H2O. These results, along with heat capacities for borax and tincalconite determined by DSC and molar volumes determined by XRPD, were used to assess the thermodynamic properties of borax dehydration as a function of temperature and pressure. The resulting phase diagrams are consistent with geologic observations of Na-borate stability.