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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 (1\bar20) twin plane. Reflections of the first domain (I) with h_{\rm b}^2+k_{\rm b}^2=5n 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 = \textstyle{3\over5}\,hb,I + \textstyle{4\over5}\,kb,I, kb,II = \textstyle{4\over5}\,hb,I - \textstyle{3\over5}\,kb,I, lb,II = lb,I for the second domain. The non-crystallographic pseudosymmetry in the melilite structure is discussed.

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Reciprocal lattice projection reconstructed using the collected intensity data

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