A COMPARISON BETWEEN THE STABILITY FIELD OF A CL-RICH SCAPOLITE AND THE END-MEMBER MARIALITE

Scapolites are widespread rock-forming aluminosilicates, which appear in many metamorphic terrains and metasomatic environments. In this study the reaction

3(Na_0.8Ca_0.2)(Al_1.2Si_2.8)O₈ + 0.8NaCl + 0.2CaCO₃ = (Na_3.2Ca_0.8)(Al_3.6Si_8.4)O₂₄(Cl_0.8(CO₃)_0.2)

was investigated to determine the effect of the Ca-rich meionite component on the Na end-member marialite. This study was done in a piston-cylinder press and internally heated gas vessels. The conditions were 500-850°C and 0.4-2.0 GPa. The starting materials were the synthetic phases scapolite of the composition marialite₈₀meionite₂₀ (Ma₈₀Me₂₀), plagioclase of the composition albite₈₀anorthite₂₀ (Ab₈₀An₂₀), halite, and calcite. Thus far, intermediate composition scapolite shows a wide thermal stability and is stable at temperatures higher than 750°C at 0.4 GPa and 760°C at 2.0 GPa. Based on experimental brackets from Almeida & Jenkins (GSA, 2015) for the water-free system involving end-member marialite and end-member meionite from Newton and Goldsmith (ZK, 1976) one can see that the intermediate composition scapolite from this study is more stable (lower thermal stability) than either end-members.

The stability of intermediate scapolite in the presence of variable amounts of H₂O was also investigated. It appears that solid solution scapolite is quite tolerant to water; it requires a minimum bulk salinity of the brine of approximately 0.2 X_NaCl at 830°C and 680°C at pressures of 2.0 GPa and 1.5 GPa, respectively, in order for it to be stable. In comparison, pure marialite is very intolerant to water; it requires a minimum bulk salinity of the brine of about 0.8 X_NaClat 1050°C and 1000°C at pressures of 2.0 GPa and 1.5 GPa, respectively, in order for it to be stable (GSA, 2015).

The intermediate scapolite investigated in this study indicates that a small change in the chemical composition from end-member marialite to Ma₈₀Me₂₀ lowers the stability field of marialite by 260°C at 2.0 GPa to 730°C by the high entropy of mixing in scapolite. This ability to be stable at much lower brine concentrations and temperatures indicates that scapolite is a mineral equivalent to mid- to high-grade metamorphic conditions (greenschist-amphibolite facies), and would be common in metasomatic environments, as it is widely reported in the literature.