Manganese speciation in Mn-rich CaCO3: A density functional theory study

Abstract

The manganese content of aragonitic bivalve shells is a potential archival indicator of temporal Mn bioavailability in aquatic environments. The Mn speciation mechanism in biogenic aragonite minerals remains elusive because the analog is challenging to synthesize, and the metastable phase has yet to be fully resolved experimentally. In this study, we performed density functional theory (DFT) computations of hypothetical Mn-doped aragonite to examine its local coordination structure and thermodynamic and electronic properties. Our DFT calculations reproduced the experimental crystal structures and solubility product constants (Ksp) of Mn-doped calcite. The magnetic moment of Mn was close to 5 μB in both Mn-doped calcite and Mn-doped aragonite (Ca1−xMnxCO3). The calculated Ksp of Mn-doped aragonite was higher than that of Mn-doped calcite and increased with Mn content, indicative of the unfavorable coprecipitation of Mn with the aragonite phase. We found that the incorporation of a small mole fraction of Mn into aragonite created significant structural distortion around the Mn site, resulting in mixed coordination numbers of Mn (mainly five and seven). Valence-to-core X-ray emission spectroscopy (XES) measurements are useful in determining the coordination environment of Mn complexes. We calculated theoretical XES spectra, with a 1s core hole in Mn, for Mn-doped calcite and four versions of Mn-doped aragonite. The Boltzmann-averaged spectrum for different coordination numbers in Mn-doped aragonite was akin to an experimental XES spectrum of aragonitic bivalve shells. The energy position of the Kβ2,5 band was calculated to be insensitive to Mn speciation in CaCO3; however, the band intensity was relatively sensitive to Mn speciation. The XES spectrum intensity decreased exponentially with increasing Mn–O distance. This quantitative XES relationship we report can reduce uncertainties in the spectral interpretation due to the absence of an Mn-doped aragonite reference spectrum.