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.