On the structural relations of malachite. I. The rosasite and ludwigite structure families.

The crystal structures of malachite Cu(2)(OH)(2)CO(3) and rosasite (Cu,Zn)(2)(OH)(2)CO(3), though not isotypic, are closely related. A previously proposed approach explaining this relation via a common hypothetical parent structure is elaborated upon on the basis of group-subgroup considerations, leading to the conclusion that the aristotype of malachite and rosasite should crystallize in the space group Pbam (No. 55). An ICSD database search for actual representatives of this aristotype leads to the interesting observation that the structure type of ludwigite (Mg,Fe)(2)FeO(2)BO(3), which is adopted by several natural and synthetic oxide orthoborates M(3)O(2)BO(3), is closely related to the proposed malachite-rosasite aristotype and thus to the malachite-rosasite family of hydroxide carbonates M(2)(OH)(2)CO(3) in general. Relations within both structure families and their analogies are summarized in a joint simplified Bärnighausen tree.