Effect of sulfate on magnesium incorporation in low-magnesium calcite

The incorporation of magnesium and sulfate in calcite is frequently used to characterize and trace the environmental conditions occurring during carbonate mineral formation. Although both ions are simultaneously incorporated in the growing calcite, the effect of sulfate on magnesium incorporation in calcite is still under-explored. In this study, we examine the Mg incorporation in low-Mg calcite as a function of growth rate at 25 °C and 1 bar pCO₂ in the presence and absence of aqueous sulfate. The obtained results suggest that high calcite growth rates induce a significant increase in the partitioning coefficient of Mg between the precipitated low-Mg calcite and aqueous solution (i.e. D_Mg= [Formula presented] ). Obtained D_Mg values exhibit similar dependence to mineral growth rate for experiments performed in the presence and absence of sulfate, however a systematic shift to lower D_Mg values is observed for calcites formed in the presence of sulfate. The lower D_Mg values of calcites formed in sulfate-bearing solutions are attributed to the incorporated SO₄ ions which provoke expansion in the unit cell along the c-axis of the newly formed calcite. A larger unit cell is unfavorable for the substitution of Ca by smaller Mg ions in calcite. The coupled effect of (i) sulfate uptake during calcite growth and (ii) precipitation rate on D_Mg can be expressed by the equation D_Mg=0.03726-0.02345×-logr_p-7-X_SO4×0.004607+0.002109×-logr_p-7; (−8 ≤ log(r_p) ≤ −7; 0 ≤ Xso₄ ≤ 2.6; T = 25 °C) and is valid for 0.01 ≤ D_Mg ≤ 0.04. In analogy to Mg also SO₄ ions are inhibiting calcite growth, thus promoting the formation of aragonite due to adsorption phenomena and blocking of calcite surface sites. These results improve our understanding on physicochemical parameters controlling CaCO₃ composition and mineral polymorphism and are discussed in their relevance for the use of magnesium and sulfate environmental proxies in natural surroundings.