How Structure Compactness Impacts the Quantitative Colour Research of Turquoise

The structure compactness has an important impact on the quantitative colour research of turquoise. The authors first characterized the colour heterogeneity of turquoise in certain areas by using a standard colour variable:CIE 1976 colour difference (ΔE). The structure compactness of turquoise sample was measured by hydrostatic weighing method for bulk property and atomic force microscope (AFM) for surface property. The loose turquoise sample shows larger ΔE and standard deviation, bigger roughness, and much smaller specific gravity value than that of the compacted sample. The loose turquoise sample also exhibits large uncertainties in the chemical composition. By using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), the authors found that the concentrations of colour causing elements such as Cu and Fe had substantial decline along with the laser ablation time, which never happen in the compacted sample. Meanwhile, the study on unearthed ancient turquoise samples shows that loose turquoise has bigger colour difference, lower anti-corrosion and anti-contamination ability than that of compacted turquoise. In contrast, turquoise with compacted structure has better quality, and presents larger specific gravity, lower surface roughness, more homogeneous colour, more uniform distribution of colour causing elements, and stronger anti-contamination ability. Therefore, the authors propose that accurate description of compactness is an indispensable criterion for the further quantitative colour classification of turquoise.