(P,T)-behavior of an intermediate scapolite of unusual I4/m symmetry

Scapolites are a group of open-framework aluminosilicates with general formula M4T12O24A, where M are monovalent or divalent cations (usually Na, Ca and minor K), T are trivalent or tetravalent cations (mostly Al and Si) in tetrahedral coordination, and A are anionic elements or groups such as Cl, CO3 and SO4. Samples with a composition closer to the Na4Al3Si9O24Cl and Ca4Al6Si6O24CO3 end-members are usually reported to crystallize with a structure described in the I4/m space group, whereas intermediate members are reported to show a P42/n symmetry. Scapolites are usually formed under metamorphic conditions in the presence of fluids, but are also reported as constituents of upper mantle xenoliths [1, 2]. Scapolites form a complex non-binary solid solution as a function of the (NaCl)-(CaCO3)-(CaSO4) substitutions, which couples with Al-Si re-arrangements [3, 4]. The variable crystal chemistry and symmetry of these minerals influences their elastic and structural response to pressure and temperature variations and, consequently, their stability fields. In this study, we have investigated the behavior of an intermediate scapolite of the composition (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01]) and its unusual symmetry I4/m, by means of in situ X-ray and neutron diffraction at high-T (from -100 to +1000 °C), high-P (up to 17.8 GPa) and combined high-T and P (up to 650 °C and 16 GPa). Experimental data show that the unusual I-centered lattice is always preserved, whereas a phase transition towards a triclinic polymorph, that can be described according to an unconventional I-1 symmetry, was found to occur at 9-10 GPa, with a modest influence from temperature. The deformation mechanisms, acting at the atomic scale, that lead to the structural instability and phase transition have been described on the basis of a series of single-crystal structure refinements. A comparison with data from literature allows to model the elastic response of scapolites as a function of their crystal chemistry, whereas a model of the thermo-elastic behavior is less straightforward on the basis of the available data, suggesting that a thorough re-investigation of the thermal behavior of the complex scapolite solid solution is desirable. References [1] J.K. Porter, H. Austrheim Terra Nova. 2017, 29, 29-35. [2] L. Torró, R.F. Martin, D. Schumann, J. Cox, S. Galì Medina, J.C. Melgarejo Draper Eur. J. Mineral. 2018, 30, 45-59. [3] D.K. Teerstra, B.L. Sheriff Chem. Geol. 1997, 136, 233-260. [4] E.V. Sokolova, F.C. Hawthorne Can. Mineral. 2008, 46, 1527-1544.