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

Abstract

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 $R\overline{3}c$ 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 α_a = 5.08 × 10⁻⁶ K⁻¹ and α_c = 18.06 × 10^–6 K⁻¹, as well as α_{Vunit cell} = 28.49 × 10^–6 K⁻¹. The geometry of (MnO₆) octahedron as function of temperature was also determined as α_Mn–O = 12.14 × 10⁻⁶ K⁻¹ and α_O–Mn–O ≈ 0.05°/100 K. The anisotropy of MnCO₃ (α_a/α_c = 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.