Effect of growth rate on nickel and cobalt incorporation in aragonite

The incorporation of Ni and Co in aragonite was experimentally investigated as a function of mineral growth rate using the constant addition technique at 25 degrees C and 1 bar pCO(2). The distribution coefficients of Ni and Co (i.e., D-Me = (Me/Ca)(solid)/(a(Me)(2+)/a(Ca)(2+))(fluid)) exhibit a linear correlation with mineral growth rate in the range -9.1 <= Log r(p) <= -7.5 mol/m(2)/s. The obtained results suggest a stronger dependency of D(Ni )to growth rate compared to that observed for D-Co. These dependencies can be described as: Log D-Ni = 1.247 (+/- 0.152) Log r(p) + 7.448 (+/- 1.212); R-2 = 0.82 Log D-Co = 0.312 (+/- 0.047) Log r(p) + 1.664 (+/- 0.383); R-2 = 0.81 The DMe-aragonite values for both Ni and Co are systematically lower than unity and their increase with increasing growth rate is in agreement with the incorporation of elements incompatible with the host mineral structure. Using the dependency of D-Ni and D-Co to the saturation indices (SI) of the reactive solution with respect to aragonite, the D-Me under equilibrium were estimated. These estimates yield equilibrium Log D-Me values of -3.9 and - 1.0 for Ni and Co, respectively. These experimentally defined D(eq )values are 1.3 to 4.3 orders of magnitude lower compared to theoretical estimates that were earlier published in the literature. Similar to other incompatible elements, the observed increase of D(Ni )and D-Co values with degree of saturation, points towards incorporation related to the density of defect sites on the mineral surface. Finally, the dependencies recorded in this study suggest that D-Ni and D-Co have the potential to be used as a proxy for saturation degree of the reactive solution.

Automated summary

The incorporation of Ni and Co in aragonite was investigated as a function of mineral growth rate. The distribution coefficients of Ni and Co showed a stronger correlation with growth rate for Ni compared to Co. These coefficients can be used to estimate the equilibrium distribution coefficients.