High-pressure and high-temperature behaviors of intermediate scapolite by in situ synchrotron X-ray diffraction

Scapolites represent a complex solid solution of volatiles-bearing aluminosilicate minerals common in metamorphic environments, from the hydrothermal to the granulite facies conditions. The three end members of the solid solution are: marialite (Na4Al3Si9O24Cl), meionite (Ca4Al6Si6O24CO3) and silvialite (Ca4Al6Si6O24SO4). Scapolites usually form for the reaction of plagioclase with salts-bearing metamorphic fluids at non-ambient pressure and temperature and, therefore, may act as reservoirs for volatiles down to the lower crust [1], or induce fluids release if destabilized [2]. To understand and predict the relative stability of a scapolite of a given crystal chemistry in a mineral assemblage at metamorphic conditions, it is crucial to determine the thermo-elastic constants across the solid-solution members, as well as the crystal structure response to varying pressure and temperature. In this light, we have investigated the high-P (at ambient-T) and high-T (at ambient-P) behaviors of a natural sample of scapolite, chemical formula (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01] intermediate between the end members marialite and meionite, by means of in situ single-crystal and powder X-ray diffraction, using both conventional and synchrotron X-ray sources. At high pressure, the investigated scapolite undergoes a phase transition, between 9.23 and 9.87 GPa, from the tetragonal I4/m space group (stable at room conditions) toward a triclinic polymorph, characterized by a highly distorted tetrahedral framework. The elastic behavior of the tetragonal scapolite has been described by fitting the experimental V-P data to a III-order Birch-Murnaghan equation of state, which provided a refined KV0 = 70(2) GPa (V0 = 0.0143(4) GPa-1) and KV = 4.8(7). A comparison with the refined bulk moduli reported in the literature for three further members belonging to the marialite-meionite joint [3,4], allows to define, at a first approximation, a chemical dependence of the scapolite bulk modulus: KV0 (GPa) = 53(2) + 0.45(4)*(%Me), where %Me = 100*[M2+/(M+ + M2+)], M = Na+,Ca2+,K+... An anomalous compressibility along the [001] axis and a change in the structure deformation mechanisms at the atomic scale suggest a destabilization at P > 3 GPa (at ambient-T). Preliminary analysis of the high-T experimental data revealed a dramatic anisotropy of the thermal expansion, which is only accommodated within the (hk0) plane, whereas for the c unit-cell parameter no T-induced deviations from the ambient value are observed within the experimental uncertainty, in agreement with what previously reported for other scapolite solid solution members [5,6]. [1] J. Hammerli, A.I.S. Kemp, N. Barrett, B.A. Wing, M. Roberts, R.J. Arculus, P. Boivin, P.M. Nude, K. Rankenburg Chem. Geol. 2017, 454, 54-66. [2] J.K. Porter, H. Austrheim Terra Nova 2017, 29, 29-35. [3] R.M. Hazen, Z.D. Sharp Am. Mineral. 1988, 73, 1120-1122. [4] P. Comodi, M. Mellini, P.F. Zanazzi Eur. J. Mineral. 1990, 2, 195-202. [5] G. Graziani, S. Lucchesi Am. Mineral. 1982, 67, 1229-1241. [6] J. Baker Am. Mineral. 1994, 79, 878-884.