Upper-pressure stability and thermochemical properties of synthetic marialite and its application to calculating paleobrine NaCl concentrations

Marialite (Na3Al3Si9O24·NaCl), the chloride end-member of the scapolite mineral group, is important for determining the chloride content of a paleofluid from which Cl-bearing scapolite formed as well as its potential role in the geochemical cycling of chlorine in tectonically active zones. Experiments on the upper-pressure stability of synthetic marialite breaking down to jadeite + coesite + halite at high pressures were done using a multi-anvil press in the range of 2.6–4.0 GPa and 1050–1250 °C for durations of 1–48 h. The boundary of this reaction was located along a straight line running from 2.8 GPa and 1050 °C to 3.5 GPa and 1250 °C. These data were then combined with data on the lower-thermal stability of marialite from this study and from previous work, along with published thermophysical data on synthetic marialite, to derive thermodynamic data for end-member marialite. The resultant 298 K and 1 atm value for the enthalpy of formation ( $$\varDelta{H}_{f}^{\text{o}}$$ ) is -12,164.03 ± 4.7 kJ/mol and for the third-law entropy (S°) is 760.34 ± 3.8 J/K·mol for a volume of 329.47 cm3/mol. These values provide good fits to the experimental data. The relatively high-temperature, high-pressure, and extremely high-salinity conditions needed to form end-member marialite means that albitite, rather than marialite, will tend to form at near-surface hydrothermal conditions. Scapolite is stabilized to typical intermediate-grade metamorphic conditions because of substantial solid solution of marialite with other scapolite components (i.e., meionite and silvialite). Thermochemical data presented here can be used to determine the NaCl content of the paleobrine that formed Cl-bearing scapolite in, for example, the Dana Hill metagabbro of the northwest Adirondack Mountains, New York, which was found to be in the range of 32–56 wt% NaCl equivalent.