STRUCTURAL MODULATIONS IN A PSEUDO-CUBIC ALUMINATE SODALITE, CA-8[AL12O24](CRO4)(2)

Complex satellite reflections in aluminate sodalite, Ca-8[Al12O24](CrO 4)(2), arise from incommensurate modulations of the framework oxygen-a tom positions in several directions, e.g. [001], [110], and [112]; the ir periodicities differ with direction. The orientational ordering of the [Ca-4 . CrO4](6+) clusters influences the positional ordering of t he framework oxygen atoms. This cluster ordering accounts for the two well-defined framework oxygen-atom positions (O1 and O2) in nosean and lazurite, and a similar type of framework oxygen-atom ordering probab ly occurs in the cubic aluminate sodalite and produces the modulated s tructure. The clusters are disordered together with the framework oxyg en atoms at higher temperature.