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 Na 4.7 K 2.6 Ca 0.1 Si 6.1 Al 5.9 O 24 (SO 4 ) 0.8 · 2H 2 O. Upon heating, the cell parameters increased in the range 25–226 °C, following paths which were best fitted by the equations a = −1 × 10 −6 T 2 + 4 × 10 −4 T + 12.821 and c = −6 × 10 −7 T 2 + 2 × 10 −4 T + 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 H 2 O and CO 2 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 H 2 O 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.