Intermediate scapolite: behavior at non-ambient conditions and unusual symmetry

The scapolite series of minerals represents a complex non-binary solid solution, which end members are: marialite [Na4Al3Si9O24Cl], meionite [Ca4Al6Si6O24CO3] and silvialite [Ca4Al6Si6O24SO4]. The members which composition falls on the marialite-meionite joint appears to be the most common in natural occurrences [1,2]. The members close to marialite on one side and to meionite on the other side, are usually reported to crystallize in the tetragonal I4/m space group, whereas intermediate scapolites are usually found in the primitive space group P42/n. In this study, we report a scapolite of intermediate composition (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01], which, based on both X-ray and neutron single-crystal diffraction data, shows an anomalous I-centered lattice (Figure 1), possibly due to anti-phase domains too small to be detected by diffraction techniques. The behavior at non-ambient conditions of the same sample has been investigated at high-P (ambient-T) by single-crystal XRD at the former ID09 beamline of ESRF (Grenoble) and at high-T (ambient-P) by powder XRD at the MCX beamline of the Elettra synchrotron (Trieste), providing the following thermodynamic parameters: V0 = 0.0143(4) GPa-1 and αV0 = 1.87(4)·10-5 K-1, respectively, which confirm that compressibility and thermal expansivity increase, along the solid solution series, from meionite to marialite [3-6]. A P-induced phase transition towards a triclinic polymorph has been observed at 9.87 GPa at ambient-T. An in situ single-crystal XRD experiment at combined high P and T (using a resistive-heated DAC), performed at the P02.2 beamline of the Petra-III synchrotron (Hamburg), allowed to detect the occurrence of the same phase transition at 10.51 GPa at 650 °C.