Chemical Bonding and Electronic Structures of Azurite and Malachite by X-Ray Photoelectron Spectroscopy

Azurite [Cu₃ (CO₃)₂ (OH)₂] and malachite [Cu₂CO₃(OH)₂] are important basic copper carbonate-hydroxide minerals that serve as historical pigments, archaeological corrosion products, and indicators of geochemical processes in oxidized copper deposits. While Fourier-transform infrared microscopy (FTIR) and Raman spectroscopy are widely employed for their identification through characteristic vibrational modes, electronic structure information derived from surface-sensitive analysis remains underexplored for these minerals. This study provides a comprehensive characterization of the chemical bonding and electronic structures of azurite and malachite through X-ray Photoelectron Spectroscopy (XPS), with particular emphasis on distinguishing subtle differences between these chemically similar minerals. High-resolution spectra of Cu 2p, C 1s, O 1s core levels and valence band regions were acquired using a Kratos AXIS Ultra spectrometer equipped with a monochromatic Al X-ray source, analyzing powdered mineral samples from South Australia and Western Australia. The Cu 2p core-level analysis revealed the most diagnostic spectroscopic differences. Azurite exhibits a single dominant Cu²⁺ photoelectron peak at 934.6 eV with two shake-up satellites, confirming homogeneous divalent copper. Malachite displays a more complex spectrum containing both Cu⁺ (932.9 eV, 2.0% observed) and Cu²⁺ (935.1 eV, 11.9% observed) components with three shake-up satellites, indicating mixed-valence character at the mineral surface. The 0.5 eV upward shift in Cu²⁺ binding energy reflects differences in hydrogen bonding and coordination geometry. In contrast, C 1s and O 1s spectra showed remarkable consistency between the two minerals: carbonate carbon binding energies were virtually identical (289.2–289.3 eV), and carbonate and hydroxyl oxygen displayed nearly identical binding energies with only 0.1 eV differences. However, intensity ratios of oxygen species directly reflected stoichiometric differences, validating spectral assignments. Valence band spectroscopy revealed a distinctive carbonate-derived feature at ~10–12 eV binding energy with substantially stronger intensity in azurite, reflecting its higher carbonate content. This study establishes XPS as a powerful complementary analytical method to FTIR and Raman spectroscopy, offering electronic fingerprinting for distinguishing azurite and malachite in complex multi-component systems. The surface-sensitive analysis uniquely detects redox processes and mixed-valence character unavailable through vibrational spectroscopy. These findings advance fundamental understanding of electronic structure in these historically important minerals and provide robust diagnostic criteria for authentication and conservation assessment of cultural heritage artifacts, archaeological corrosion products, and geochemical interpretation of weathered copper-bearing materials.