On the symmetry of wulfenite (Pb[MoO₄]) from Mežica (Slovenia)

Wulfenite [lead(II) molybdate(VI)] is known as a scheelite structure in the I4₁/a space group. The structure of the unusual `hemimorphic' wulfenite crystals from the Mežica mine was refined in the noncentrosymmetric space group I using a Pb/Mo exchange disorder model with the approximate composition Pb_0.94Mo_0.06[MoO₄]. Pb atoms in the 2b positions are substituted by Mo at about 12%. The crystal is shown to be twinned by inversion. Hemimorphism may result from the short-range chemical ordering of the metal atoms at the 2b positions.