Thermal expansion of cancrinite

I n t r o d u c t i o n THE structures of the cancrinite-group minerals are character ized by parallel s ix-membered rings consisting of alternating A104 and SiO4 tetrahedra. The hexagonal symmetry is the result of the stacking of such six-membered rings in an AB... sequence. This stacking leads to large continuous channels that are formed by twelve-membered rings of alternating A1Oa and SiO4 tetrahedra (Fig. la). These are among the largest channels known for zeolites. The cancrinite structure also consists of small cages, known as ecages in zeolite chemistry. The e-cage is also known as cancrinite cage or 'undecahedral' cage, which is bounded by 5 six-membered and 6 four-membered rings and, thus, the e-cage is also known as 11-hedral cage (Fig. lb). The e-cages occur along the three-fold axes (Fig. la). Crystal-chemical data for a few common members of the cancrinite group are given (Table 1). The stacking of six-membered rings in these cancrinites is the AB... sequence (Fig. l b). In general, the e-cages contain atoms or clusters of atoms that have a net charge difference of 1+ valence units. For example, [Na.H20] + clusters occur in the e-cages of cancrinite, Mineralogical Magazine, December 1996, Vol. 60, pp. 9 Copyright the Mineralogical Society hydroxy-cancrinite, a n d vishnevite, but [Ca.C1] § clusters occur in davyne. The e-cage in microsommite contains an Na cation, while tiptopite, the beryllophosphate analogue of hydroxy (basic) cancrinite, contains a K cation (Table 1). The other interstitial cations and anions occur in the large channels that are parallel to the 63 axes (Fig. la). Microsommite has a c parameter that is similar to that of cancrinite. However, its unit cell is three times (Z = 3) that of cancrinite, with am = ~/3 %. Other variations in the a cell parameter occur in pitiglianoite and quadridavyne (Table 1). The sodalite-group minerals have strong structural similarities to the cancrinite-group minerals, but the former have an ABC... stacking sequence that leads to cubic instead of hexagonal symmetry. This sequence leads to an offset of the C-type layer and causes a network of large 13-cages in the sodalitegroup minerals. Thermal expansion studies were made on many sodalite-gronp minerals and their structures were also modeled at elevated temperatures (e.g. Demsey and Taylor, 1980; Henderson and Taylor, 1978; Hassan and Grundy, 1984a, 1991a); however, similar studies were not done on cancrinite or other members of the 949-956