Incorporation of Co in dolomite structure: coupled single crystal and EPMA investigations of cobaltoan dolomites from Tenke-Fungurume, D.R. of Congo

Cobaltoan dolomite from Tenke-Fungurume, D.R. of Congo, can host up to 20% mol Co (Barton et al., 2014). Incorporation of Co in dolomite is definitely more relevant than in cobaltoan calcite, because of the closeness in ionic radius between Mg2+ (0.72) and Co2+ (0.745) (Shannon, 1976). A set of crystals from this locality was studied through coupled EPMA and single crystal structural refinements to investigate structural changes induced by Co incorporation in dolomite structure. As expected, Co incorporation reflects in a clear direct trend in cell volume expansion, with the a cell parameter more sensible than the c cell parameter to Co content. Dolomite structure can be described (Reeder & Wenk, 1983) through “layers” of Me2+ alternating with “layers” hosting CO3 groups along the c axis, approximating an hexagonal close packing. Both the Mg and Ca polyhedra are trigonally distorted, namely elongated along the threefold axis. A scrutiny of Me-O bond distances shows a direct correlation between the volume of (Mg,Co) coordination polyhedron and the unit cell volume, whereas the volume of Ca polyhedron is inversely related to the Co content hosted in Mg polyhedron. In fact, when the (Mg,Co) polyhedron expands, the oxygen atom is shifted towards Ca cation, resulting in a contraction of the Ca coordination polyhedron. The distortion of Ca and (Mg,Co) coordination polyhedra is less pronounced in “basal” edges, namely those polyhedron edges lying perpendicular to the c axis, respect to “lateral” edges. The expansion of the “basal” edges of the Mg polyhedron with increasing Co content is more relevant than the slight expansion of the Mg lateral edges; the same holds for the contraction of Ca polyhedron. This is in agreement with the pronounced variation of the a cell parameter, directly related to the length of the “basal” edges. In dolomites, because of the difference in length between the longer Ca-O and shorter (Mg,Co)-O bonds, a “rotation” of the octahedra and of the CO3 groups along the ternary axis, is necessary to accomplish a fit. Their orientation in calcite structure, hosting Ca octahedra all of the same size, can be assumed as starting zero point for comparison. Following Reeder & Wenk (1983) the mean rotation angle of CO3 groups is a convenient measure of the net rotations of all the components: in cobaltoan dolomites the net rotation angles of CO3 groups decreases with increasing Co content.