The thermal behaviour and dehydration of pitiglianoite, a mineral of the cancrinite-group

This paper relates the in situ X-ray diffraction and FTIR high-T study of pitiglianoite from Monte Cavalluccio (Campagnano, Rome, Italy), with composition Na4.7K2.6Ca0.1Si6.1Al5.9O24(SO4)0.8 · 2H2O. Upon heating, the cell parameters increased in the range 25–226 °C, following paths which were best fitted by the equations a = -1x10-6T2 + 4x10-4T + 12.821 and c = -6x10-7T2 + 2x10-4T + 5.264, for a and c, respectively. In the range 226–403 °C, there was a strong decrease of both a and c, while for T > 403 °C there was a new expansion of the structure. The FTIR data showed a major loss of both H2O and CO2 in the range 200–400 °C. The structure of pitiglianoite at room-T, before the heating run, was characterised by columns of base-sharing undecahedral cages, containing chains of alternating Na cations and H2O molecules, and by large channels containing sulphate groups as well as Na and K ions. Structure refinement at 457 °C showed that all water was released from the cages and that significant extra-framework K (1.4 apfu) migrated from the large channel into the cages, throughout the hexagonal tetrahedral rings. A corresponding amount of Na cations migrated from the cages into the channels; a rotation of the sulphate groups, due to the presence of Na instead of K between two sulphate groups was also observed. Refinement at room-T after the experiment showed that the thermally-induced changes were substantially irreversible. The re-hydration of pitiglianoite was prevented by the Na <=> K cation exchange within the pores due to the thermal treatment.