short communications
Additional reflections seemingly leading to a tetragonal fivefold supercell have been observed in a number of melilites. A careful examination of the collected intensity data reveals that this feature is due to the coexistence of two twin domains related by a () twin plane. Reflections of the first domain (I) with overlap those of the second domain (II) when both the following additional conditions are verified: -hb + 2kb = 5n and 2hb + kb = 5n. Indices of the superimposed reflections are hb,I kb,I lb,I for the first domain, and hb,II = hb,I + kb,I, kb,II = hb,I - kb,I, lb,II = lb,I for the second domain. The non-crystallographic pseudosymmetry in the melilite structure is discussed.
Supporting information
Portable Document Format (PDF) file https://doi.org/10.1107/S0108768103000041/bk0126sup1.pdf |