Discovery of Ge2+ in quartz: Evidence from EPR/XAS experiments and DFT calculations, and implications for Ge/Si systematics

Abstract

The Ge/Si systematics as a biogeochemical tracer with diverse applications from paleo-climatic reconstructions to discrimination of magma sources and elucidation of Earth’s early evolution hinges on the coherent behavior of these elements in the tetravalent state. However, determination of Ge speciation in quartz and other silicate minerals is technically challenging because this element almost invariably occurs at several parts per million or lower concentrations. This contribution reports a detailed study of Ge speciation in quartz by combining single-crystal electron paramagnetic resonance spectroscopy, synchrotron X-ray absorption spectroscopy, and ab initio theoretical calculations. Our single-crystal and powder electron paramagnetic resonance spectra of artificially irradiated quartz reveal a suite of previously reported Ge electron centers (GECs) such as [GeHLi₂]⁰ and [GeHH₂]⁰ centers as well as a new GeHLi center. These multiply-compensated GECs in artificially irradiated quartz suggest that their precursors before irradiation involve the diamagnetic Ge²⁺ state. Ge K-edge X-ray absorption spectroscopic data of selected quartz samples further support the presence of Ge²⁺. Theoretical calculations reproduce the experimental ¹H and ⁷Li hyperfine constants of the [GeHLi₂]⁰ center and suggest the new GeHLi center to be a new variant of the multiply-compensated GECs with the second monovalent cation in a distant c-axis channel, again supporting the Ge²⁺ state. The presence of Ge²⁺ in sedimentary-diagenetic quartz, in particular, challenges existing thermodynamic data that Ge⁴⁺ is the only stable oxidation state in aqueous solutions under near-surface conditions. Incorporation of Ge²⁺ in quartz and other silicates can significantly affect Ge/Si fractionations, with important implications for their applications as a biogeochemical tracer from surficial environments to magmatic-hydrothermal systems, Earth’s core-mantle differentiation, and other planetary processes.