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 yea r. A synthetic, mixed Ca-Mg carbonate (41.7 mol% MgCO3 and with no obs ervable ordering reflections on X-ray diffraction patterns) was recrys tallized in solutions with ionic strengths similar to seawater in two sets of time series experiments. Dolomite recrystallization reaction r ates were initially rapid, but slowed significantly with duration of t he experiments. Reaction rates were highly temperature dependent. Dolo mite completely recrystallized within 286 hours at 200 degrees C, wher eas less than 30% recrystallization was attained in 336 days at 50 deg rees C. Increases in mol% MgCO3 of the recrystallized dolomites were i nitially rapid, but slowed with extent of reaction. Despite complete r ecrystallization at 200 degrees C, a stoichiometric dolomite was never achieved (a maximum of 48.6 mol% MgCO3 was attained). Unit cell dimen sions, measured by X-ray diffraction, decreased with increasing extent s of recrystallization and largely responded to changes in stoichiomet ry. Increases in cation ordering during recrystallization lagged behin d 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 recrys tallization. Strontium distribution coefficients, D-Sr = (Sr/Ca)(recry stallized dolomite)/(Sr/Ca)(solution), ranged from a maximum of 0.22 ( 8% recrystallization) at 50 degrees C to a minimum of 0.044 (100% recr ystallization) at 200 degrees C. D-Sr varied primarily as a function o f the extent of recrystallization, probably due to thermodynamic effec ts such as variable stoichiometry and, to a lesser extent, cation orde r of the dolomite. Likewise, Na contents of dolomites decreased with i ncreasing temperature and degree of recrystallization. The most signif icant decrease in Na concentrations occurred rapidly suggesting that N a may be a sensitive indicator of the early recrystallization process.