PRECIPITATION OF ND-CA CARBONATE SOLID-SOLUTION AT 25-DEGREES-C

The formation of a Nd-Ca carbonate solid solution was studied by monit oring the reactions of calcite with aqueous Nd, orthorhombic NdOHCO3(s ) with aqueous Ca, and calcite with hexagonal Nd-carbonate solid phase as a function of time at 25-degrees-C and controlled pCO2 (g). All ex periments reached steady state after 200 h of reaction. The dominant m echanism controlling the formation of the solid solution was precipita tion of a Nd-Ca carbonate phase from the bulk solution as individual c rystals or at the orthorhombic NdOHCO3(s)-solution interface. The lack of Nd adsorption or solid solution at the calcite-solution interface suggests that the solid solution was orthorhombic and may be modeled a s a mixture of orthorhombic NdOHCO3(s) and aragonite. Orthorhombic NdO HCO3(s) was determined to be the stable Nd-carbonate phase in the Nd-C O2-H2O system at PCO2(g) 0.1 and 1.0 atmospheres at 25-degrees-C. The equilibrium constant corrected to zero ionic strength for orthorhombic NdOHCO3(s) solubility is 10(10.41(+/-0.29)) for the following: NdOHCO 3(s) + 3H+ = Nd3+ + CO2(g) + H2O.