Partially crystalline silica varieties in gemmology: The case study of the blue Andean opal

Opal is an amorphous or poorly crystalline hydrated form of silica (usually given as SiO2·nH2O) occurring in veins and nodules of various colours. The term opal defines several sub-categories of aggregates with varying structural orders, from a glass-like structure to packed silica nano-spheres or nanograins [1]. Opal's intrinsic and peculiar complexities enhance its desirable aesthetic aspect, appreciated in gemmology and jewellery. A well valued and popular opal variety is the blue Andean opal, from the Acari Mine (Arequipa Department, Peru) (Fig1). The distinctive blue hue of this variety is commonly ascribed to the presence of copper-rich mineral inclusions [2,3,4,5]. In this study, a multi- methodological approach has been used to investigate the features of the blue Andean opal. X-Ray diffraction methods have been used for the characterization of the mineralogical assemblage, along with the quantification of the amorphous fraction, showing the challenge in distinguishing among opal subcategories due to their blurred boundaries. Raman mapping highlighted different silica phases and the distribution of impurities within the matrix (Fig.2). The microstructure of the blue Andean opal resulted emblematic of the precipitation processes of the silica aggregate (Fig.3). The analysis of trace elements concentration provided information about the potential composition of the original fluids, from which the Andean opal precipitated. Notably, this research highlighted that the blue colouration of the Andean opal is ascribable to the presence of a mineralogical inclusion, but it is also influenced by the combined effect of factors closely dependent on the geological processes and environment of formation.