Crystal Structure of Thecotrichite, an Efflorescent Salt on Calcareous Objects Stored in Wooden Cabinets Nanna Wahlberg, † Tomc ̌ e Runc ̌ evski, ‡ Robert E. Dinnebier,* ,‡ Andrea Fischer, § Gerhard Eggert, § and Bo B. Iversen † † Department of Chemistry and iNANO, Center for Materials Crystallography, Langelandsgade 140, Aarhus C 8000, Denmark ‡ Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany § State Academy of Art and Design Stuttgart, Am Weißenhof 1, 70191 Stuttgart, Germany * S Supporting Information ABSTRACT: The crystal structure of thecotrichite, Ca 3 (CH 3 COO) 3 Cl(NO 3 ) 2 ·6H 2 O, an efflorescent salt occur- ring 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 (P2 1 /a, Z = 4, a = 23.5933(4), b = 13.8459(3), c = 6.8010(1) Å, β = 95.195(2)°, V = 2212.57(7) Å 3 ) 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. ■ INTRODUCTION The formation of efflorescent salts onto surfaces of porous calcareous objects has been a problem known for over a century. The efflorescence on shells, later called Byne’s disease, was first reported in 1896. 1,2 Thirty years later, it was found that acetic acid is the main contributor to the surface damage. 3 Considerable amounts of acetic acid vapor are emitted from wood (especially oak), used to make storage cabinets and display cases. During the pioneering research on this degradation process, two compounds were found to be among the major products: calclacite, Ca(CH 3 COO)Cl· 5H 2 O, whose crystal structure was reported, 4 and “efflorescence X”, whose exact composition and structure have remained ambiguous ever since. 5 Efflorescence X was later renamed as thecotrichite and became one of the most frequently observed efflorescent salts on rare ceramic and limestone objects in museums (one example is given in Figure 1a,b). 4,6,7 Gibson et al., using a combination of powder diffraction, ion chromatog- raphy, infrared spectroscopy, NMR, and thermogravimetry, reported thecotrichite ’ s composition to be Ca 2.95 (CH 3 COO) 2.91 Cl 0.97 (NO 3 ) 2.03 ·6.55H 2 O. 8 The chemical formula was simplified to Ca 3 (CH 3 COO) 3 Cl(NO 3 ) 2 ·7H 2 O, and the actual amount of water in the structure remained uncertain. Crystal structure determination of this important efflorescent salt failed, presumably due to the unavailability of single crystals of quality suitable for single crystal diffraction (thecotrichite crystallizes in the form of microcrystalline needles, as shown in Figure 1c,d). However, the micro- crystallinity presents no obstacle for powder X-ray diffraction (PXRD), and after careful grinding thecotrichite becomes amenable to structural investigations. The limited structural knowledge on thecotrichite has severely hindered its identification and characterization. Visually, it is very difficult to discriminate it from other calcium acetates (and calcites), as neither its white color nor needle-like morphology is unique. Its powder diffraction signature was only communicated, but without any report on the crystal structure. 8,9 Identi fication by spectroscopic methods is challenging and ambiguous (as these efflorescent salts some- times occur in mixtures and they might have similar spectra). 8 Traditionally, the presence of thecotrichite on affected surfaces has been only speculated, based on visual inspection and on knowledge of the storage history of the suffered object. Herein, we present the PXRD pattern, detailed crystal structure solution and Rietveld refinement, and the results of Raman spectroscopic investigations and thermal analyses (TG and DTA) performed on this frequently observed efflorescent salt. PXRD can easily identify and quantify crystalline phases with Received: February 9, 2015 Revised: April 29, 2015 Published: May 6, 2015 Article pubs.acs.org/crystal © 2015 American Chemical Society 2795 DOI: 10.1021/acs.cgd.5b00197 Cryst. Growth Des. 2015, 15, 2795-2800