Thermal and combined high-temperature and high-pressure behavior of a natural intermediate scapolite

A natural intermediate member of the scapolite solid solution (Me47; experimental chemical formula: (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01]), with the unusual I4/m space group, has been studied at various temperatures and combined high-T and high-P by means of in situ single-crystal and powder X-ray diffraction, at both conventional and synchrotron sources. In addition, single-crystal neutron diffraction data were collected at ambient-T and 685 °C. A fit of the experimental V-T data with a thermal equation of state yielded a calculated thermal expansion coefficient at ambient conditions: αV25°C = 1/V0·(∂V/∂T)P,25°C = 1.74(3)·10-5 K-1. A comparative analysis of the elastic behavior of scapolite based on this study and other high-T XRD data reported in the literature suggests that a thorough re-investigation of the different members of the marialite-meionite solid solution is needed to fully understand the role of crystal chemistry on the thermal behavior of these complex non-binary solid solutions. The experimental data obtained within the full temperature range of analysis at ambient pressure confirm that the investigated sample always preserves the I4/m space group, and possible implications on the metastability of I4/m intermediate scapolite are discussed. Neutron diffraction data show that no significant Si and Al re-arrangement among the T sites occurs between 25 and 685 °C. The combined high-T and high-P data show that at 650 °C and between 10.30(5) and 10.71(5) GPa a phase transition towards a triclinic polymorph occurs, with a positive Clapeyron slope (i.e., dP/dT > 0). A comprehensive description of the atomic-scale structure deformation mechanisms induced by temperature and/or pressure, including those leading to structural instability, is provided based on single-crystal structure refinements.