The thermal behaviour and dehydration of pitiglianoite, a mineral of the cancrinite-group Elena Bonaccorsi a, * , Giancarlo Della Ventura b , Fabio Bellatreccia b , Stefano Merlino a a Dipartimento di Scienze della Terra, Universita ` di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy b Dipartimento di Scienze Geologiche, Universita ` Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy Received 27 June 2006; received in revised form 4 September 2006; accepted 6 September 2006 Available online 7 November 2006 Abstract 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 Æ 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 expan- sion 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. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Pitiglianoite; Cancrinite-group; X-ray diffraction; FTIR spectroscopy; Dehydration; Thermal behaviour 1. Introduction Pitiglianoite [1,2] is a rare mineral of the cancrinite–soda- lite group, structurally and chemically very close to zeolites. It is characterised by layers of six-membered rings of ordered SiO 4 and AlO 4 tetrahedra stacked along [0 0 1] so as to form a three-dimensional framework. Different stack- ing sequences give rise to different mineral species [3]; piti- glianoite in particular is built up by a simple ABAB ... stacking sequence which is characterised by the develop- ment along [0 0 1] of open channels (denoted as [6 6 12 2/2 ] in the IUPAC nomenclature [4]) and columns of base-sharing cages (denoted as [4 6 6 5 ] in the IUPAC nomenclature [4] but also known as e-cages or undecahedral cages). The open channels of pitiglianoite host Na and K cations, as well as sulphate anions, while the undecahedral cages contain chains of alternating Na cations and water molecules [1]. In the large channels, the sulphate anion is located at the same level of three sodium cations, which alternate along c with three K + cations [1,2]; this ionic ordering in the chan- nel along c can be realised in two different ways, related by a 2 1 axis parallel to c. The two distinct types of channels may be distributed in the a, b plane with various degrees of order. In pitiglianoite such ordering of the channels is actu- ally observed, and superstructure reflections occur corre- sponding to a super = p 3Æa. In this case, the channels at [1/ 3, 2/3, z] and [2/3, 1/3, z] of the supercell show exactly the two types of ordered sequences, just related by 2 1 axis, 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.09.040 * Corresponding author. Tel.: +39 050 2215 704; fax: +39 050 2215 830. E-mail address: elena@dst.unipi.it (E. Bonaccorsi). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 99 (2007) 225–235