Thermophysical properties of synthetic marialite

Abstract

Marialite (Na₃Al₃Si₉O₂₄·NaCl) represents a key end-member of the scapolite mineral group because it has the potential for revealing the chloride content of the paleofluid from which it formed. Here we provide measurements of the basic thermophysical properties of synthetic marialite which do not currently exist and which complement similar data for calcium-carbonate-bearing scapolites. Synthetic marialite was made from reagent oxides and NaCl treated at 1050 °C and 1.7 GPa for 48–120 h. Average unit-cell dimensions for synthetic marialite at 298 K and 1 atm are a_o = 12.038 ± 0.002 Å, c_o = 7.539 ± 0.004 Å, and V_o = 1092.6 ± 0.8 ų, with a molar volume of 328.99 ± 0.24 cm³/mole. Thermal expansion measurements were made at 1 atm from 298–1105 K and showed that a increases while c decreases with an overall increase in volume upon heating. Compressibility measurements were made at room temperature in a diamond-anvil cell using 4:1 methanol: ethanol pressure medium in transmission mode at the Cornell High Energy Synchrotron Source facility with pressures ranging from 1 atm to 9.6 GPa. The a dimension is more compressible than c up to ~ 5 GPa, beyond which there is noticeable softening along the c axis. Equation of state modeling was done on the combined pressure–temperature-volume data using a Tait equation of state yielding bulk modulus and thermal expansion values for K_o, K’, and α of 51.0 ± 2.0 GPa, 6.68 ± 0.83, and 2.75 ± 0.17 × 10^–5/K, respectively. Compared with other scapolite data in the literature, the marialite (Na₃Al₃Si₉O₂₄·NaCl)-meionite (Ca₃Al₆Si₆O₂₄·CaCO₃) join behaves similarly to the albite-anorthite plagioclase join, with end-member marialite having the highest thermal expansion and lowest bulk modulus along the compositional join.