English: Variation in calcium speciation with bicarbonate activity and pH. Calcium is thermodynamically favored to exist as dissolved ions in the regions shaded blue and within minerals in the regions shaded yellow. The scatter point and dashed lines illustrate how bicarbonate production and hydrogen ion consumption by microbial reactions can cause calcite to become supersaturated in a solution with a temperature of 25°C. At the scatter point, the solution has a pH of 6 and contains -2 and -4 log activities of calcium and bicarbonate, respectively. From there, microbial reactions can cause calcite to saturate by increasing pH to just under 7 or by increasing the log activity of bicarbonate to just under -2. Many microbial reactions can cause both changes, creating a diagonal reaction path to saturation. Further increases in bicarbonate activity and/or pH would work to progressively supersaturate calcite, increasing the likelihood of precipitation. This activity diagram was calculated with The Geochemist’s Workbench software (Bethke, 2018).