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.

中文翻译：

---

无杂质菱锰矿 (MnCO3) 的晶体结构和热膨胀特性

为了解释之前 Rao 和 Murthy (J Mater Sci 5: 82, 1970) 报道的菱锰矿 (MnCO3) 中热膨胀特性的异常各向异性，Li 等人。(High Temp High Press, 2019)，晶体结构的评价被认为是矿物学中不可或缺的一个重要方面。本着这种精神，在高压-温度 (P-T) 条件下合成了尺寸高达 100 μm 的无杂质菱锰矿单晶。通过单晶 X 射线衍射 (XRD) 研究标准晶体结构，不含天然样品常见的杂质。为 R3¯c\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage 获得的晶胞参数{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$R\overline{3}c$$\end{document} 对称性是 a = 4.7754(5) Å 和 c = 15.6484(18) Å，最终 R 值为 0.0162。(MnO6) 八面体表现出异常键角，更趋向于正八面体的 90°，这与 MgCO3、FeCO3 和 CaCO3 的键角完全不同。使用 100 到 370 K 的单晶 XRD，热膨胀系数被量化为 αa = 5.08 × 10−6 K−1 和 αc = 18.06 × 10–6 K−1，以及 αVunit cell = 28.49 × 10 –6 K-1。(MnO6) 八面体的几何形状作为温度的函数也被确定为 αMn-O = 12。14 × 10−6 K−1 和 αO–Mn–O ≈ 0.05°/100 K。MnCO3 的各向异性 (αa/αc = 3.55)，类似于 MgCO3 (~ 3.0, Markgraf and Reeder, Am Mineral, 70 : 590–600, 1985)，表明键角的差异对热膨胀性能没有显着影响。根据端元（MgCO3、FeCO3、MnCO3和CaCO3）的标准晶体结构，证明方解石型结构中的阳离子取代与刚体模型一致，线性固溶关系与天然的线性固溶关系高度一致。碳酸盐。