Beyond Optical Basicity: Role of O–Cl Bond Formation in CaCl2-Induced Silicate Network Disruption in Weathered Biotite

Abstract

The optical basicity framework has long served as a guiding principle for predicting chemical reactivity in silicate systems, yet it fails to explain a striking experimental observation: despite its extremely low optical basicity, CaCl₂ uniquely induces the phase transformation of weathered biotite (WB) at 600 °C–700 °C, whereas high-basicity additives such as CaO and CaCO₃ do not. Here, we address this paradox by combining Cl K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, thermal desorption spectroscopy (TDS), and molecular orbital (MO) calculations. Cl K-edge NEXAFS indicates the coexistence of Cl^– and positively polarized O-coordinated Cl environments, while TDS shows Cl-containing desorption products consistent with the formation of oxychlorine-containing species below the onset of phase transformation. In contrast, NaCl produces no detectable O–Cl bonding species and fails to induce structural transformation under vacuum conditions. MO calculations indicate that O–Cl bonds destabilize silicate frameworks by weakening Si–O–Si linkages more effectively than Si–Cl bonds. These findings suggest that O–Cl bond formation provides a fundamentally distinct mechanism for low-temperature silicate network disruption beyond conventional optical basicity theory.