Recrystallization of dolomite: An experimental study from 50-200 degrees C

The recrystallization of dolomite was investigated experimentally from 50 degrees to 200 degrees C for durations up to approximately one year. A synthetic, mixed Ca-Mg carbonate (41.7 mol% MgCO3 and with no observable ordering reflections on X-ray diffraction patterns) was recrystallized in solutions with ionic strengths similar to seawater in two sets of time series experiments. Dolomite recrystallization reaction rates were initially rapid, but slowed significantly with duration of the experiments. Reaction rates were highly temperature dependent. Dolomite completely recrystallized within 286 hours at 200 degrees C, whereas less than 30% recrystallization was attained in 336 days at 50 degrees C. Increases in mol% MgCO3 of the recrystallized dolomites were initially rapid, but slowed with extent of reaction. Despite complete recrystallization at 200 degrees C, a stoichiometric dolomite was never achieved (a maximum of 48.6 mol% MgCO3 was attained). Unit cell dimensions, measured by X-ray diffraction, decreased with increasing extents of recrystallization and largely responded to changes in stoichiometry. Increases in cation ordering during recrystallization lagged behind increases in mol% MgCO3. Significant increases in cation order were only observed in the 200 degrees C experiments. Coprecipitation of Sr with dolomite varied as a function of temperature and degree of recrystallization. Strontium distribution coefficients, D-Sr = (Sr/Ca)(recrystallized dolomite)/(Sr/Ca)(solution), ranged from a maximum of 0.22 (8% recrystallization) at 50 degrees C to a minimum of 0.044 (100% recrystallization) at 200 degrees C. D-Sr varied primarily as a function of the extent of recrystallization, probably due to thermodynamic effects such as variable stoichiometry and, to a lesser extent, cation order of the dolomite. Likewise, Na contents of dolomites decreased with increasing temperature and degree of recrystallization. The most significant decrease in Na concentrations occurred rapidly suggesting that Na may be a sensitive indicator of the early recrystallization process.