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We have investigated the structure of Co$_2$CO$_3$(OH)$_2$ using DensityFunctional Theory (DFT) simulations as well as Powder X-Ray Diffraction (PXRD)measurements on samples synthesized via deposition from aqueous solution. Weconsider two possible monoclinic phases, with closely related but symmetricallydifferent crystal structures, based on those of the minerals malachite androsasite, as well as an orthorhombic phase that can be seen as a common parentstructure for the two monoclinic phases, and a triclinic phase with thestructure of the mineral kolwezite. Our DFT simulations predict that therosasite-like and the malachite-like phases are two different local minima ofthe potential energy landscape for Co$_2$CO$_3$(OH)$_2$, and are practicallydegenerate in energy, while the orthorhombic and triclinic structures areunstable and experience barrierless transformations to the malachite phase uponrelaxation. The best fit to the PXRD data is obtained using a rosasite model(monoclinic with space group P1121/n and cell parameters a = 3.1408(4) {\AA}, b= 12.2914(17) {\AA}, c = 9.3311(16) {\AA}, $\gamma$ = 82.299(16)$^{\circ}$).However, some features of the PXRD pattern are still not well accounted for bythis refinement and the residual parameters are relatively poor. We discuss therelationship between the rosasite and malachite phases of Co$_2$CO$_3$(OH)$_2$and show that they can be seen as polytypes. Based on the similar calculatedstability of these two polytypes, we speculate that some level of stackingdisorder could account for the poor fit of our PXRD data. The possibility thatCo$_2$CO$_3$(OH)$_2$ could crystallize, under different growth conditions, aseither rosasite or malachite, or even as a stacking-disordered phaseintermediate between the two, requires further investigation.

Crystal structure of cobalt hydroxide carbonate Co$_2$CO$_3$(OH)$_2$: density functional theory and X-ray diffraction investigation