Formation and solid solution behavior of Ca-rhodochrosites in marine muds of the Baltic deeps

Authigenic Ca-rhodochrosites are found in organic-rich sediments in the deep anoxic basins of the Baltic Sea. Rhodochrosite formation is apparently the result of organic matter degradation. The rhodochrosite contains 10 to 40 mol% CaCO₃ and 2 to 5 mol% MgCO₃, as determined by microprobe. The composition of the carbonates at a given depth varies by about 5 mol% MnCO₃. SEM revealed the rhodochrosite to be present as globular aggregates consisting of microcrystallites. Dissolution features such as cavities within the aggregates are frequently observed and indicate that extensive recrystallization takes place. Pore waters are greatly supersaturated with respect to both rhodochrosite and calcite. Rhodochrosite is only found in sediments where pore waters show the highest degree of supersaturation. Due to ion exchange, Ca²⁺ diffuses from the underlying freshwater clays. This results in increasing a_Ca²⁺ a_Mn²⁺ ratios in the pore waters from the sediment surface downward. In response to this, the maximum CaCO₃ MnCO₃ ratio also increases with depth. The composition of rhodochrosite which forms under these conditions may be determined by both the porewater a_Ca²⁺ a_Mn²⁺ and precipitation kinetics. However, if their composition is controlled solely by the a_Ca2+ a_Mn2+ in solution, their behavior can be described by a regular solid solution model.