Structural variations in Co-rich dolomite from the Tenke-Fungurume district, Democratic Republic of Congo, have been examined with coupled EPMA and single-crystal structure refinements for Co contents up to 0.30 atoms per formula unit. Full occupation of Ca at the A site is constantly found, with progressive Co incorporation at the B structural site leading to isotropic expansion of the B coordination octahedra and a corresponding anisotropic contraction of the A polyhedra, mainly achieved through the shortening of “basal” polyhedral edges. The simultaneous opposing variation of AO6 and BO6 polyhedra explains the strongly anisotropic expansion of the unit-cell parameters.

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