title: Phase transformation of calcium oxalate dihydrate-monohydrate: effects of relative humidity and new spectroscopic data.
abstract: New data on vibrational properties of calcium oxalates and their controversial transformation mechanism are presented. We have focused on whewellite (CaC2O4·H2O) and weddellite [CaC2O4·(2+x) H2O], the most common phases of calcium oxalate; these compounds occur in many organisms, in kidney stones and in particular kinds of films found on the surface of many works of art. Low temperature experiments carried out by Fourier transform infrared spectroscopy have highlighted both the high structural order in the crystalline state of whewellite and the disordered distribution of the zeolitic water molecules in weddellite. The synthesised nanocrystals of weddellite have been kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the role of "external" water molecules on their stability. Moreover, in order to identify the different kinds of water molecules, a re-investigation, supported by quantum chemical calculations, of the observed vibrational spectra (IR and Raman) of whewellite has been conducted.
authors.affiliation: Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, CNR, Via Cozzi 53, 20125 Milano, Italy. Electronic address: c.conti@icvbc.cnr.it.
authors.forename: Claudia
authors.initials: C
authors.lastname: Conti
authors.affiliation: Dip. Scienze Ambiente, Territorio e Scienze della Terra, Università degli Studi di Milano Bicocca, Piazza della Scienza 1, 20126 Milano, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Casati
authors.affiliation: Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, CNR, Via Cozzi 53, 20125 Milano, Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Colombo
authors.affiliation: Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, CNR, Via Cozzi 53, 20125 Milano, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Realini
authors.affiliation: Dip. Chimica, Materiali e Ingegneria Chimica "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.
authors.forename: Luigi
authors.initials: L
authors.lastname: Brambilla
authors.affiliation: Dip. Chimica, Materiali e Ingegneria Chimica "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Zerbi
mesh_terms: D002129:Calcium Oxalate / Q000737:chemistry*; D006813:Humidity; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013059:Spectrum Analysis, Raman / Q000379:methods; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D002129:Calcium Oxalate
keywords: FTIR spectroscopy; Quantum chemical calculation; Raman spectroscopy; Weddellite; Whewellite; X-ray diffraction