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Department of Geology, Faculty of Science, University of Kuwait, P.O. Box 5969, Safat. Kuwait, 13060.

ABSTRACT

Transmission electron microscopy (TEM) shows ordering of [Ca₄.CrO₄]⁶⁺ clusters in an alumi­nate sodalite, Ca8[Al₁₂O2₄₁(CrO₄)². Four distinct phases occur for this aluminate sodalite and they have orthorhombic, tetragonal, modulated. and cubic structures. Above 610 K, the [Ca4.CrO4]⁶⁺ clusters in the cubic (space group 143m) aluminate sodalite are disordered together with the framework oxygen atoms over the O1 and O2 positions. Between 453 and 610 K, the [Ca₄^.CrO₄]⁶⁺ clusters are ordered and cause two-dimensional periodic APBs along {110} planes. The formation of these APBs is accompanied by positional modulations of the framework oxygen atoms on the O1 and O2 positions. The periodic APBs Arc about 54Ǻ apart. The complex satellite reflections in the modulated phase arise from incommensurate modulations of the framework oxygen-atom positions in several directions and are prominent along <001>, <110>, and <112>; their periodicities differ with direction. The different sizes of the [Ca₄.CrO₄]⁶⁺ clusters and their orientational ordering influence the positional ordering of the framework oxygen atoms. This cluster ordering accounts for the two well-defined framework oxygen-atom positions (O1 and O2) in nosean and ]azurite and similar type of framework oxygen-atom ordering probably occurs in the cubic aluminate sodalite. Between 432 and 453 K, the [Ca₄.CrO₄]⁶⁺ clusters are ordered differently and the symmetry changes from a modulated structure to a tetragonal superstructure. The tetragonal superstructure changes to an orthor­hombic superstructure with different ordering of [Ca₄^.CrO₄]⁶⁺ clusters below 432 K. Structural modulations occur during either or both of the above transitions. The transition from tetragonal to orthorhombic symmetry is associated with random transformation twins on a {110} plane; this plane is both the compositional and twin plane.