INTEGRATED AND MULTI-SCALE CHARACTERISATION OF GEM MATERIALS LACKING LONG-RANGE ORDER. THE CASE STUDIES OF CHRYSOCOLLA AND ANDEAN BLUE OPAL

The PhD research project was focused on the integrated and multi-scale characterisation of gem materials lacking long-range structural order. These materials, positioned between crystalline and completely amorphous systems, present significant challenges in terms of classification and characterisation, due to their intrinsic heterogeneity and complexity, which conventional analytical techniques are often unable to investigate properly. However, these characteristics are often responsible for the particularly attractive aesthetic qualities of these materials, which also determine their value in the field of gemmology. The study aimed to address some unresolved issues in the field of gemmology concerning two varieties of material lacking a long-range structural order, chrysocolla and Andean blue opal. Chrysocolla is a hydrated copper silicate (Cu2-xAlx(H2-xSi2O5)(OH)4·nH4O), known for its vivid blue–green colour and its millennial use (Bianucci et al., 2009; Clark, 2021; Torpy et al., 2021). Its classification remains debated (both in mineralogy and gemmology) because of its variable composition and uncertainties regarding its crystal structure (Frost and Xi, 2013; Torpy et al., 2021; Dold et al., 2023; Kahou et al., 2025). Andean blue opal is a particular gemmological variety of hydrated silica (SiO2·nH2O), extracted in southern Peru and of strong cultural value since pre-Hispanic times (Hyršl, 2001; Caucia et al., 2015). It is appreciated for its intense colours, ranging from milky blue to green, but the causes of its colouration, as well as its structural and compositional features, are still not fully understood (Gaillou et al., 2008b; Caucia et al., 2015). The in-depth study of chrysocolla was carried out through a multidisciplinary and multiscale approach, made necessary by the complex and strongly heterogeneous nature of the mineral. The methodological strategy involved the integration of conventional techniques, commonly used in the study of crystalline materials (e.g., X-ray diffraction), with advanced approaches specifically suited to nanocrystalline or amorphous systems, such as X-ray Total Scattering and transmission electron microscopy (TEM). Given the spatial heterogeneity of the samples, spot analyses at the micrometric scale were also adopted, including micro-Raman spectroscopy and electron microprobe analysis in wavelength-dispersive mode (EMPA-WDS) (using a multivariate analysis approach (PCA) for in-depth investigation of chemical data). The experimental protocol was structured as a progressive workflow across different scales of investigation, from macroscopic observations to nanoscale characterisations, ensuring a hierarchical and integrated view of the mineralogical system. The selected samples, from two reference deposits – Capo Calamita mine (Elba Island, Italy) and Cornwall mine (Pennsylvania, USA) – were investigated in terms of chemical composition, microstructure, degree of crystallinity, and associations with other phases involved in the formation process. Particular attention was paid to the identification of potential compositional fingerprints of chrysocolla, while at the same time addressing the analytical challenges posed by its nature and optimising the analytical procedures. Finally, thermodynamic models of the Cu–SiO2–SCOH system (T = 298 K, P = 1 atm) were developed to define the stability fields of the mineral, providing insights into the chemical and physical conditions favourable to its genesis. The investigation of Andean blue opal was developed through a multiscale and integrated strategy, aimed at combining mineralogical characterisation, textural observations, and detailed chemical analyses. A preliminary petrographic study made it possible to identify the main textural features of the Andean blue opal and to highlight the distribution of mineral inclusions, which served as a basis for further investigation. Mineralogical characterisation was carried out using high-resolution X-ray diffraction, integrated with micro-Raman spectroscopy, in order to define the opaline matrix and the accessory phases present. Raman mapping and X-ray microtomography (micro-CT) were used to investigate the spatial distribution of inclusions and their relationships within the silica matrix. Compositional analysis was carried out at different levels: EMPA-WDS for the determination of major and minor elements, and LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) for the quantification of trace elements, thus providing a detailed and quantitative picture of the chemical variability. Finally, the data obtained were subjected to multivariate statistical analysis (PCA), useful for highlighting internal heterogeneities and possible correlations between composition, inclusions, and genetic processes. The integration of these methodologies made it possible to build a coherent and hierarchical picture of the studied variety, addressing the complexity of opal through a structured approach.