Strontium isotope fractionation during precipitation of strontianite in aqueous solutions as a function of temperature and reaction rate

In order to study Strontium (Sr) isotope fractionation during the precipitation of strontianite (SrCO3) as a function of the specific precipitation rate (R*) and temperature (T), strontianite was precipitated at 12.5, 25.0 and 37.5 degrees C by diffusing NH3 and CO2 gases into aqueous solutions. The specific precipitation rate R* (mol/cm(2).h) for every sample was determined by applying the initial rate method. The mean isotope difference between bulk solution and precipitate (Delta Sr-88/86(strontianite-solution)) was found to be 0.279 +/- 0.005% (2 sigma(mean)) independent of both rate and temperature. Hence, Sr isotope fractionation in strontianite is completely different from that in calcite and aragonite, where a strong dependency from both rate and temperature can be observed. The latter is interpreted to reflect the competition between Sr2+ and Ca2+ ions for incorporation into the calcium carbonate crystal lattice, which is absent during the precipitation of pure strontianite. The isotope difference between strontianite and bulk solution then simply reflects the intermolecular forces in the aqueous solutions as well as the kinetic effect. The difference in the (Delta Sr-88/86(strontianite-solution) between experiments then reflects the dehydration energy of Sr ions in the adsorption layer of SrCO3.

Automated summary

The study on Strontium isotope fractionation during the precipitation of strontianite revealed that the process is independent of specific precipitation rate (R*) and temperature (T) unlike calcite and aragonite. The difference in isotope composition between strontianite and bulk solution is mainly influenced by intermolecular forces in aqueous solutions and kinetic effects.