Transmission electron microscopy (TEM) has provided direct evidence for ordering of [Ca 4 .CrO 4 ] (super 6+) clusters in a synthetic aluminate sodalite, Ca 8 [Al 12 O 24 ](CrO 4 ) 2 . The ordering of the above clusters causes antiphase domains that are separated by two-dimensional periodic antiphase boundaries (APBs). Such APBs are rare in minerals and inorganic solids, but they do occur in alloys. The APBs are at 90 degrees to each other, parallel to {110} planes, and separated by about 54 Aa. At high temperature, the [Ca 4 .CrO 4 ] (super 6+) clusters in the aluminate sodalite (space group I43 m) are disordered, together with the framework oxygen atoms. As the temperature is lowered, the [Ca 4 .CrO 4 ] (super 6+) clusters are ordered and cause two-dimensional periodic APBs that are probably accompanied by positional modulations of the framework oxygen atoms.

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