Influence of Water on Turquoise Quality Characterized bySpectroscopy under Variable Temperatures

Turquoise is a hydrous copper, aluminum phosphate mineral, and the content of the adsorbed water, crystal water and structural water in turquoise is closely related to its compactness and structure,which also contribute to its texture and the quality of colour. In this paper,infrared spectroscopy with curve fitting was used to study the changes in water features, in order to detect the quality of turquoise changing under the heat treatment. The turquoise with different textures and colours from Ma'anshan were chosen as our studying samples. The temperature of the heat treatment was set from 25-400℃ and the adsorbed water, crystal water, and structural water gradually escaping from turquoise when heating were characterized by infrared(IR) spectra and ultraviolet-visible(UV-Vis) spectra. According to curve fitting method, the areas of IR band for absorbed water and the IR band for crystal water were integrated. The change of colour coordinate values during the heat treatment was also measured by UV-Vis spectroscope. Combining the results of IR and the colour coordinate, the conclusion are:(1) The intensity of water related peaks in loose turquoise (bad quality) are higher than those in dense turquoise sample (good quality);(2)In the dense turquoise samples, the water features are more stable than those in the loose turquoise samples;(3) The blue turquoise are denser than the blue-green to green turquoises, and it is less likely to lose water under low temperature and change colour.