STABILITY FIELD OF THE CL-RICH SCAPOLITE MARIALITE

Scapolites are widespread rock-forming aluminosilicates. Marialite is the Cl-rich end member of the group. Scapolite appears in metasomatic environments and many metamorphic terrains. Even though Cl-rich scapolite is presumably stable over a wide range of pressure and temperature little is actually known about its stability field. In this experimental study the reaction 3*NaAlSi₃O₈ + NaCl = Na₄Al₃Si₉O₂₄Cl was established.

The experiments were performed using a piston-cylinder press and internally heated gas vessels. The temperature and pressure conditions range from 700-1050 °C and 0.5-2.0 GPa, respectively. The starting materials were synthetic phases including end-member marialite, high-albite, and halite, and were treated dry (non-hydrothermal). Thus far, marialite appears to be stable at temperatures higher than 920 °C at 0.65 GPa and 990 °C at 2.0 GPa, marialite is not stable over the temperature range of 800-950 °C at 0.5 GPa.

Marialite has potential as a geothermometer and geobarometer. Establishing the reaction boundary above, where ΔG=0, enables the extraction of thermochemical data for marialite, such as, enthalpy, and entropy, for which there are no extant data. From this study estimated values for the enthalpy and entropy at 298 K and 1 atm have been calculated as -12,184.1 ± 1.5 kJ/mole and 0.7369 ± 0.0012 kJ/K*mole, respectively, based on data for high-albite and halite of Holland and Powell (2011, JMG).

It appears that pure marialite requires temperatures greater than 900 °C and a minimum depth of 18 km for its formation, which indicates that it is a mid to deep crustal mineral. Scapolites of intermediate composition with meionite could be stable to lower pressure and temperature conditions. The stability of marialite proposed by Filliberto et al., (2014, EPSL) has a similar slope to the results of this experimental study, however it is approximately shifted to lower temperatures by 90 °C. This study is the first experimentally determined stability for marialite.