gems, mineralogy, sodalite, lapis lazuli

Complex satellite reflections in aluminate sodalite, Ca₈Al₁₂O₂₄₂, arise from incommensurate modulations of the framework oxygen-atom positions in several directions, e.g. ⟨001⟩, ⟨110⟩, and ⟨112⟩; their periodicities differ with direction. The orientational ordering of the [Ca₄·CrO₄]⁶⁺ clusters influences 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 lazurite, and a similar type of framework oxygen-atom ordering probably occurs in the cubic aluminate sodalite and produces the modulated structure. The clusters are disordered together with the framework oxygen atoms at higher temperature.