A natural intermediate member of the scapolite solid solution {Me47; 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, using both conventional and synchrotron X-ray 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 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 previous high-T XRD data 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 nonbinary 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 rearrangement 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 toward 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.

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