Sr, Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate

Abstract

Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr 2+, Co 2+, Mn 2+ and Cd 2+in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations Ω = 1 to Ω = 5.5. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: log λ Sr = 0.249 log R −1.57 log λ Mn = −0.266 log R + 1.35 log λ Co = −0.173 log R + 0.68 log λ Cd = −0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO 3 per mg seed crystal per min. In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions. Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.