Crystal chemistry and stability relations in the system MgCO3–NiCO3*

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Abstract
Hydrothermal syntheses and recrystallization reactions in the temperature range 100–400°C confirm the existence of a sizeable immiscibility gap between MgCO3 and NiCO3. Crystallization of a coprecipitated basic magnesium-nickel carbonate gel at 100°C resulted in the initial formation of a single-phase rhombohedral carbonate of intermediate composition, similar to natural magnesian gaspeite. However, longer reaction times at 100°C or reaction at higher temperatures produced topotactic two-phase assemblages. Furthermore, the synthetic single-phase material readily recrystallized to a topotactic two-phase assemblage at 200°C. These results indicate that solid solutions of intermediate compositions such as natural magnesian gaspeite may form metastably at low temperatures but that they are thermodynamically unstable with respect to unmixing.

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