Contribution to the Understanding of the Colour Change in Bluish-Grey Limestones
Teresa P. Silva,
Daniel de Oliveira,
João P. Veiga,
Vitor Lisboa,
Jorge Carvalho,
M. Alexandra Barreiros,
Mathilda L. Coutinho,
Eduardo Salas-Colera,
Rogério Vigário
Affiliations
Teresa P. Silva
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal
Daniel de Oliveira
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal
João P. Veiga
Department of Conservation and Restoration, CENIMAT/i3N, Materials Research Centre, NOVA School of Science and Technology, FCT NOVA, 2829-516 Caparica, Portugal
Vitor Lisboa
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal
Jorge Carvalho
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal
M. Alexandra Barreiros
LNEG (National Laboratory for Energy and Geology), Materials for Energy Unit, Estrada do Paço do Lumiar 22, 1649-038 Lisboa, Portugal
Mathilda L. Coutinho
Laboratório Hercules—IFAA, Universidade de Évora, Largo Marquês de Marialva 8, 7000-809 Évora, Portugal
Eduardo Salas-Colera
Spanish CRG BM25 SpLine Beamline, The European Synchrotron (ESRF), 71 Av. des Martyrs, 38000 Grenoble, France
Rogério Vigário
MVC-Mármores de Alcobaça, Ataíja de Cima, 2460-713 Alcobaça, Portugal
Bluish-grey limestones have been extensively used as ornamental stones for decoration purposes in buildings, as well as in works of art, and accordingly, have been the target of intense exploration. In Portugal, the Jurassic limestone massif known as the Maciço Calcário Estremenho (MCE), has been the source of grey-coloured ornamental stones, namely the Azul Valverde (one of the most well-known bluish-grey limestones) and Atlantic Blue varieties, both of which may undergo colour changes in outdoor environments. In this sense, it is important to understand the sudden colour change from bluish-grey to yellow/beige in the same limestone block in a quarry, or even, what happens to the colour when polished limestone is placed outdoors. This study was undertaken using various techniques, namely XRF (X-ray fluorescence spectrometry), XRD (X-ray diffraction), SEM (scanning electron microscopy), DTA–TG (differential thermal analysis/thermogravimetry) and colourimetry. Synchrotron radiation was also used at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) where XANES (X-ray Absorption Near Edge Structure) spectra at Fe K-edge were collected to ascertain the speciation state of Fe in different coloured zones of the limestone, previously checked by EDXRF (energy dispersive X-ray fluorescence). The presence of Fe2+ and Fe3+ are responsible for the greyish and yellow/brown colour, respectively. On the other hand, the UV radiation from the sun causes a quickened and severe bleaching/fading on the dark blue/grey polished limestone.