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.