Impurity-like Photoelectron Activity of Natural Silicates: Multiscale Analysis Through Spectroscopic Characterization and Electrochemical Responses

Observations of photoelectric conversion in Fe- and Mn-rich semiconductor mineral coatings highlight their potential role in the origin of life and the evolution of environmental conditions. However, natural silicate minerals, which make up most of the Earth’s crust, are generally considered wide-bandgap insulators and are not expected to exhibit a photoelectric effect. In this study, we experimentally confirm measurable impurity-like photoelectron activity in natural silicate minerals and explore possible regulatory mechanisms. We show that electron-active elements (e.g., structural Fe and Ti) and lattice defects in minerals such as pyroxene and mica can reduce the optical gap (Eopt) to below ~4.13 eV, producing small photocurrents ranging from 0.010 to 0.114 μA/cm2 on ITO substrates (background signal excluded). The structural types of these minerals—chain, island, layer, and framework—may influence their photoelectric responses by affecting electron transport pathways. Notably, light wavelength strongly controls both the photoelectric relative activity (PRA = 3–10 for silicates) and the decay kinetics (0.002–0.021 s−1) of minerals. Visible light (400–800 nm) markedly enhances photocurrent densities in low-bandgap minerals such as limonite (Eopt = 2.11 eV). In contrast, ultraviolet light (UVB, 300 nm) enhances photoelectric responses in high-bandgap minerals, including feldspar and quartz (Eopt = 4.31 and 6.08 eV, respectively). Multivariate statistical analysis further indicates that elemental composition governs spectroscopic features that influence photoelectric behavior. Among these, Fe, Al, Si, and Ti are identified as key regulatory elements. These results provide new insights into the role of natural silicates in photoelectron-driven environmental and geological processes and highlight the potential of silicate-based materials for solar energy conversion applications.