Structure and colour properties in the Egyptian blue Family, M1-xM ' xCuSi4O10, as a function of M, M ' where M, M ' = Ca, Sr and Ba

Solid solutions in the Egyptian Blue Family, M1-xM'xCuSi4O10, where M, M' are the alkali-earth cations Ca, Sr and Ba, have been successfully synthesised from stoichiometric mixtures of silica, alkali-earth metal carbonates and copper carbonate at high temperature using solid state reactions. The structure of the blue products has been determined by Rietveld analysis using high quality powder X-ray diffraction data. As the average size of the cations on the alkali-earth metal site increases, the cell parameters also increase, although the variation occurs in an anisotropic manner due to the flexibility of the structure. UV-vis spectra and colour coordinate data were collected on the materials to correlate the changes in the structure with the observed colour properties. A concomitant reduction in lightness indicating potential weakening of the tinting capability is observed as the cell parameters lengthen. Prolonged exposure to natural light has a deleterious effect on the colour of these synthetic materials in a similar way to the natural minerals. (c) 2005 Elsevier Ltd. All rights reserved.