Crystal structure of impurity-free rhodochrosite (MnCO3) and thermal expansion properties

To explain the anomalous anisotropy in thermal expansion properties reported in rhodochrosite (MnCO3) previously Rao and Murthy (J Mater Sci 5: 82, 1970), Li et al. (High Temp High Press, 2019), the evaluation of crystal structure is thought to be indispensable as an important aspect in mineralogy. In this spirit, single crystals of impurity-free rhodochrosite, up to 100 μm in size, were synthesized under high-pressure–temperature (P–T) conditions. The standard crystal structure, without the impurities common to natural samples, was investigated by means of single-crystal X-ray diffraction (XRD). The unit cell parameters obtained for the R3¯c\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R\overline{3}c$$\end{document} symmetry were a = 4.7754(5) Å and c = 15.6484(18) Å, with a final R value of 0.0162. The (MnO6) octahedron exhibits an anomalous bond angle that tends more toward 90° of a regular octahedron, which is totally different from those of MgCO3, FeCO3, and CaCO3. Using the single-crystal XRD from 100 to 370 K, the thermal expansion coefficients were quantified as αa = 5.08 × 10−6 K−1 and αc = 18.06 × 10–6 K−1, as well as αVunit cell = 28.49 × 10–6 K−1. The geometry of (MnO6) octahedron as function of temperature was also determined as αMn–O = 12.14 × 10−6 K−1 and αO–Mn–O ≈ 0.05°/100 K. The anisotropy of MnCO3 (αa/αc = 3.55), similar to that of MgCO3 (~ 3.0, Markgraf and Reeder, Am Mineral, 70: 590–600, 1985), indicates that the difference in bond angle has no significant effect on the thermal expansion properties. According to the standard crystal structures of end members (MgCO3, FeCO3, MnCO3, and CaCO3), the cation substitution in calcite-type structures is proven to agree with the rigid body model and the linear solid solution relationship is highly consistent with those of natural carbonates.