Crystal Structure of Thecotrichite, an Efflorescent Salt on Calcareous Objects Stored in Wooden Cabinets

The crystal structure of thecotrichite, Ca3(CH3COO)3­Cl­(NO3)2·6H2O, an efflorescent salt occurring on surfaces of porous calcareous objects stored in wooden cabinets, was solved ab initio from high-resolution, laboratory X-ray powder diffraction data. The compound was found to contain one water molecule per formula unit less than what was previously reported. The crystal structure of thecotrichite (P21/a, Z = 4, a = 23.5933(4), b = 13.8459(3), c = 6.8010(1) Å, β = 95.195(2)°, V = 2212.57(7) Å3) consists of a network of calcium ions, connected through acetate and nitrate ions, forming a metal–organic framework. In addition, five of the six chemically different water molecules are directly coordinated to the calcium ions, with the remaining water molecule located in the interstitial space, together with the chloride ion. The needle-like morphology of the microcrystals was rationalized from the crystal structure. It is suggested that the crystallite growth mechanism depends heavily on the porous nature of the crystal structure. The thermal characteristics and stability of the material were studied. Structural and spectroscopic information on this efflourescent salt are provided to ease its characterization and identification, especially in museums and art collections worldwide.

本研究针对木质橱柜储存的多孔钙质器物表面产出的风化盐（efflorescent salt）——科特里克石（thecotrichite，化学式为Ca₃(CH₃COO)₃Cl(NO₃)₂·6H₂O），利用高分辨率实验室X射线粉末衍射数据通过从头算法（ab initio）解析其晶体结构。研究发现，该化合物每个化学式单元所含的水分子数较此前报道的结果少1个。科特里克石的晶体结构空间群为P2₁/a，晶胞分子数Z=4，晶胞参数为a=23.5933(4) Å、b=13.8459(3) Å、c=6.8010(1) Å，β=95.195(2)°，晶胞体积V=2212.57(7) ų，其结构由钙离子通过醋酸根与硝酸根离子连接形成金属-有机框架（metal–organic framework）网络构成。此外，6种化学环境各异的水分子中有5个直接与钙离子配位，剩余1个水分子与氯离子一同位于间隙空间中。微晶的针状形貌可通过该晶体结构得到合理解释，且研究表明微晶生长机制在很大程度上依赖于晶体结构的多孔特性。本研究还对该材料的热特性与稳定性进行了探究，并提供了该风化盐的结构与光谱学信息，以助力其表征与鉴定，尤其适用于全球范围内的博物馆与艺术藏品场景。