Twinning in natural melilite simulating a fivefold superstructure

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: -h_b + 2k_b = 5n and 2h_b + k_b = 5n. Indices of the superimposed reflections are h_b,I k_b,I l_b,I for the first domain, and h_b,II = h_b,I + k_b,I, k_b,II = h_b,I - k_b,I, l_b,II = l_b,I for the second domain. The non-crystallographic pseudosymmetry in the melilite structure is discussed.