Intermediate scapolite: crystal chemistry, structure and behavior at non-ambient (P,T)-conditions

Scapolites, general formula M4T12O24A, are important minerals in metamorphic rocks, ranging from the greenschists to the amphibolite facies, where can act as hosts for volatiles (mainly Cl-, CO32- and SO42- anions). From the crystal-chemical point of view, they represent a complex group of minerals, for which three end members have been described: marialite (Na4Al3Si9O24Cl), meionite (Ca4Al6Si6O24CO3) and silvialite (Ca4Al6Si6O24SO4). Along the marialite-meionite joint, complex substitution mechanisms govern the occurrence of three binary solid solutions (Sokolova & Hawthorne, 2008), which are coupled with intriguing crystallographic features. In fact, in the literature, the members close to the marialite and meionite sides are reported to crystallize in the I4/m space group, whereas the intermediate members are always reported to share the P42/n space group. In this work, a gem-quality transparent single-crystal of intermediate scapolite ((Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01]) from Madagascar has been investigated by means of both conventional lab and synchrotron X-ray diffraction. Interestingly, all the experimental X-ray diffraction datasets show the occurrence of systematic extinctions compatible with an I-centered lattice, which is in contrast to what expected for an intermediate scapolite member. The high-pressure and high-temperature behaviors of the same intermediate sample of scapolite have also been investigated by means of in situ powder and single-crystal X-ray diffraction, using conventional X-ray sources, at the University of Innsbruck (HT-SCXRD), or synchrotron facilities, at Elettra (Trieste, HT-PXRD) and at ESRF (Grenoble, HP-SCXRD). The high-P evolution of the unit-cell volume of scapolite has been fitted by a III-order Birch-Murnaghan equation of state, which yielded a refined bulk modulus of 70(2) GPa (βV0 = 0.0143(4) GPa-1). A comparison with the high-pressure behavior of three further members belonging to the marialite-meionite joint (Hazen & Sharp, 1988; Comodi et al., 1990) confirms the control played by the crystal chemistry on the bulk compressibility: at a first approximation, the bulk modulus linearly increases from marialite to meionite. In addition, a displacive phase transition from the I4/m toward a triclinic polymorph was found to occur at 9.87 GPa. Preliminary analysis of the high-temperature data revealed a significant anisotropic thermal expansion, which is almost exclusively accommodated in the plane perpendicular to the tetragonal axis, i.e. (hk0). Comodi P., Mellini M. & Zanazzi P.F. 1990. Scapolites: variation of structure with pressure and possible role in the storage of fluids. Eur. J. Mineral., 2, 195-202. Hazen R.M. & Sharp Z.D. 1988. Compressibility of sodalite and scapolite. Am. Mineral., 73, 1120-1122. Sokolova E.V. & Hawthorne F.C. 2008. The crystal chemistry of the scapolite-group minerals. I. Crystal structure and long-range order. Can Mineral, 46,1527-1554.