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Crystal structure refinement of a sepiolite/indigo Maya Blue pigment using molecular modelling and synchrotron diffraction

Roberto Giustetto, Davide Levy, Olivia Wahyudi, Gabriele Ricchiardi and Jenny G. Vitillo
Crystal structure refinement of a sepiolite/indigo Maya Blue pigment using molecular modelling and synchrotron diffraction
European Journal of Mineralogy (June 2011) 23 (3): 449-466

Abstract

Maya Blue is an artificial pigment used in Pre-Columbian America, renowned for its chemical stability. The pigment can be considered a precursor of modern inclusion compounds as a hosting microporous clay (palygorskite or sepiolite) shelters the guest indigo dye (< or =2 wt%) within its micro-channels. While most papers on Maya Blue are focused on the interaction between indigo and palygorskite, this study describes the pigment structural features when sepiolite is the host structure. Synchrotron X-ray powder diffraction patterns were collected on both pristine sepiolite and sepiolite+indigo (2 wt%) pigment. The pigment structure was investigated with the Rietveld method, basing on both molecular mechanics and the refined structure of sepiolite. The evidence obtained shows that: (i) indigo molecules, encapsulated within the micro-tunnels, stay close to a TOT strip in order to receive H-bonds from the structural OH (sub 2) ; (ii) there is no evidence for direct metal-oxygen bonds between the sepiolite Mg and the indigo C=O groups, as the applied heating (< or =190 degrees C) does not remove structural OH (sub 2) ; (iii) the indigo molecule is affected by 4-fold disorder, as it occupies only one of four partially superposed equivalent sites; (iv) indigo and the zeolitic H (sub 2) O compete to occupy the channels; refined occupancies showed that the dye fills 27 vol% of the channels whereas 73 vol% is occupied by H (sub 2) O. Calculated indigo weight % (1.9) is in close agreement with experimental data; (v) indigo encapsulation modifies zeolitic H (sub 2) O sites, increasing the number and strength of mutual hydrogen bonds; (vi) difference-Fourier maps computed removing indigo contribution confirmed the position of the molecule inside the channels.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 23
Serial Issue: 3
Title: Crystal structure refinement of a sepiolite/indigo Maya Blue pigment using molecular modelling and synchrotron diffraction
Affiliation: Universita di Torino, Dipartimento di Scienze Mineralogiche e Petrologiche, Turin, Italy
Pages: 449-466
Published: 201106
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 77
Accession Number: 2011-064958
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: Nanostructured Interfaces and Surfaces, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201136
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