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

The crystal structures of malachite Cu₂(OH)₂CO₃ and rosasite (Cu,Zn)₂(OH)₂CO₃, 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)₂FeO₂BO₃, which is adopted by several natural and synthetic oxide orthoborates M₃O₂BO₃, is closely related to the proposed malachite-rosasite aristotype and thus to the malachite-rosasite family of hydroxide carbonates M₂(OH)₂CO₃ in general. Relations within both structure families and their analogies are summarized in a joint simplified Bärnighausen tree.