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.