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 (120) twin plane. Reflections of the first domain (I) with h(b)(2) + k(b)(2) = 5n 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) = 3/5h(b,I) + 4/5k(b,I), k(b,II) = 4/5h(b,I) - 3/5k(b,I), l(b,II) = l(b,I) for the second domain. The non-crystallographic pseudosymmetry in the melilite structure is discussed.