The Role of Iron and Mg/Ca Ratio in Dolomite Synthesis at 192°C

The reaction path that dolomite follows in its formation, approach to stoichiometry, and approach to order differs in the presence or absence of iron. Dolomite was synthesized in hydrothermal bombs using eight starter combinations heated at 192°C for eight time periods. When iron was present dolomite became the dominant mineral within two days, whereas it took five days to produce similar amounts of dolomite without iron. Near-stoichiometric composition (51%-52% Ca) was reached in three days when iron was present, versus six days in iron-free experiments. Further, a better cation-ordered dolomite formed earlier in iron-containing experiments. The absence of a significant difference in the results of experiments using$Fe^{+2} vs. Fe^{+3}$suggests that the influence of iron was exerted in solution rather than in the solid phase and that oxidation state exerts relatively little influence on the formation of dolomite. More$Fe^{+2} than Fe^{+3}$enters dolomite in equivalent conditions, no doubt due to charge balance compensation of the cations. Finally, the presence or absence of iron had no obvious affect on crystal size; however, dolomite rhombs in samples prepared with iron had smoother faces than rhombs in similar samples prepared without iron. Dolomite formed more quickly, approached stoichiometry more rapidly, and became better-ordered in less time in samples prepared with$Fe^{+2}$and a$1:1 Mg^{+2}/Ca^{+2}$ratio relative to experiments with$Fe^{+3}$and a$1:2 Mg^{+2}/Ca^{+2}$ratio. Results suggest that dolomite formed initially by precipitation from solution onto a calcite substrate.