The role of arsenate in the precipitation of aragonite-type Mg- and Sr-bearing Ca‑carbonates at Earth surface conditions

Abstract

The incorporation of magnesium (Mg), strontium (Sr), and arsenic (As) into calcium carbonate minerals (CaCO₃) during precipitation is well documented. The formation of Ca‑carbonate with an aragonite-type structure is typically favoured at alkaline pH and high temperatures. However, aragonite has also been observed in As-rich environments under ambient conditions. This study investigates the influence of arsenate (As(V)) on the formation of Mg- and Sr-bearing CaCO₃ with both calcite- and aragonite-type structures. As(V)-free and As(V)-bearing carbonate samples were synthesized using the spontaneous aqueous precipitation. The Mg²⁺ and Sr²⁺ ions were added in varying concentrations, either individually (Mg²⁺ or Sr²⁺) or in combination (Mg²⁺ and Sr²⁺), to promote substitution at Ca²⁺ sites within the carbonate structure. The impact of As(V) on cation site occupancy was then assessed. The synthesized Mg- and Sr-bearing Ca‑carbonates were characterized using several analytical techniques, including Rietveld refinement of X-ray powder diffraction data, inductively coupled plasma optical emission spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy. Synchrotron-based X-ray absorption spectroscopy was also used to evaluate the As solid speciation.

The results suggest that Mg-Sr-As(V)-bearing CaCO₃ crystallizes in an aragonite-type structure, as As(V) promotes the incorporation of higher amounts of Mg and Sr into the Ca‑carbonate structure. During the precipitation process, post-nucleation adsorption of aqueous MgAsO₄⁻ and/or SrAsO₄⁻ complexes onto amorphous Ca carbonate (ACC) enhances the incorporation of Mg and Sr as ionic pairs at the solution-ACC interfaces. Subsequently, during crystal growth, As(V) species became integrated into the mineral structure, occupying sites analogous to those of carbonate ions, thereby promoting the formation of aragonite or strontianite. Furthermore, carbonates with an aragonite-type structure can accommodate higher concentrations of As compared to those with calcite-type structures.