Hydrous calcium magnesium carbonate, a possible precursor to the formation of sedimentary dolomite

Material of composition Ca1.056 Mg0.944 (SO4)0.035 (CO3)1.965 ⋅0.26H2O has been synthesised from solutions of Ca(NO3)2, MgSO4 and Na2CO3. X-ray diffraction and infrared spectroscopy show this to be identical to "protodolomite" found in nature, and that the structure is related to dolomite but is hydrous, retaining water to beyond 200°C on heating. This hydrous magnesium calcium carbonate grows as spherules, with a distinct core-rim structure apparent in optical microscopy. It seems likely that in low-temperature environments (around 80°C) a dolomite crystal grows in nature via this or a similar hydrous intermediate phase, which may hold the solution to the "dolomite problem".