The crystal structure of the zeolite offretite, K1.1Ca1.1Mg0.7[Si12.8Al5.2O36].15.2H2O

Abstract

The crystal structure of offretite (hexagonal, a= 13.291, c= 7.582/~,) from Mt. Simiouse, France, has been refined in the space group P~m2 to a residual of 0.108. The frame structure proposed by Bennett & Gard [Nature, Lond. (1967) 214, 1005 was confirmed. It comprises alternating double and single (7,:r,½) respectively, forming columns with two types of rings of six tetrahedra centred on (0,0,0) and 1 2 cavity, one similar to those found in cancrinite, the other in the centre of the double ring. The columns are cross-linked to enclose channels ca. 6.9 A wide, parallel to e, and cavities similar to those in gmelinite, with 'windows' 3.6 A diameter. Each cancrinite-type cavity contains a K + ion that cannot be replaced with an NH + ion at 80°C. During synthesis, the cancrinite cage probably collects round the K ion to form a precursor of erionite or offretite. A few (0.14) double rings contain a Ca ion. Most (0.82) gmelinite-type cavities contain an Mg ion coordinated to five water molecules. A feasible arrangement was deduced for the Ca ions and water molecules that occupy seven-tenths of the volume of the channels. 5.4 out of 15.2 H20 per unit cell were not located; they are probably in random sites or are mobile. (Si, AI)-O distances suggest that most of the 5.2 A1 atoms are located in the 12 tetrahedra of the double ring; this implies a high degree of local ordering, and the space group P~m2 is probably an average one. Disorder comprising intergrowth with erionite is discussed as this places limits on refinement, and also constricts the channels. Consideration of the unit-cell dimensions of zeolites with frames comprising rings of six tetrahedra showed that the ratio nac-1, where n is the number of rings in the c-period, can be used to indicate the presence of double rings only, single rings only, or both, in frame structures of this type.

Towards a general description of hexagonal three-dimensional framework structures: the lateral connection of trigonal columns (LCTC) group

“…OFF (Gard & Tait, 1972), see above] and it is two when the trigonal columns are symmetry related by threefold screw axes [e.g. MAZ (Rinaldi, Pluth & Smith, 1975)].…”

“…Some ambiguity has to be resolved concerning the type of trigonal column used for classifying (Gard & Tait, 1972); (b) LTL (Barrer & Villiger, 1969); (c) hypothetical LTL-related" net (Barrer & Villiger, 1969). In (b) high-membered rings (S8R) and in (c) low-membered rings (S4R) are formed between neighbouring trigonal columns.…”

“…also in OFF (offretite) (Gard & Tait, 1972)]. A trigonal chain of 2T trigonal cages (denoting polyhedral cages with threefold symmetry and two on-axis oppositely oriented T nodes) connected by forming 1-1 (or T2) units (denoting a group of two linked T nodes on a trigonal axis with a T-O-T bond angle of 180 °) can be regarded as being a trigonal column with on-axis 1-membered stacks (1MS) and off-axis 3m-membered stacks (m positive integer).…”

“…As an example: in OFF (Gard & Tait, 1972) [ Fig. 3(a)], three types oftrigonal column can be distinguished: (i) the PTC made of e and D6R cages; (ii) the secondary trigonal column (STC) consisting ofgmelinite cages; and (iii) the tertiary trigonal column (TTC) forming the 12R channel.…”

“…2T orthorhombic group (18) References" (1) Bosmans & Andties (1990); (2) Jatchow (1965); (3) Smith, Rinaldi & Dem Glasser (1963); (4) Gard & Tait (1972); (5) Staples & Gard (1959); (6) Bennett & Marcus (1987); (7) Andries (1989); (8) Bennett & Smith (1985); (9) Barrer & Villiger (1969); (10) Bennett, Cohen, Flanigen, Pluth & Smith (1983);(11) WyckotI (1925); (12) Gies (1986); (13) Gibbs (1926); (14) Gerke & Gies (1984); (15) Smith & Bennett (1981); (16) Tambuyzer (1977); (17) Bragg & Gibbs (1925); (18) ; (19) Harvey & Meier (1989). Framework structures in subgroups 1 or 7 (Table 1 a) generated by trigonal columns with 1MS's belong to the tridymite group ).…”