Synergistic Direct-Transformation–Crystallization Mechanism in the Quartz-to-Coesite Reconstructive Transition

Abstract Coesite, the first discovered high-pressure polymorph of silica, is one of the most robust geological indicators of ultrahigh-pressure metamorphism and hypervelocity impact events, yet its formation mechanism remains debated. Prior studies proposed that coesite is a retrograde phase crystallizing from silica melt or glass during decompression, whereas recent geological evidence revealed a direct solid-state transition pathway from quartz to coesite, leaving the underlying microscopic pathways unresolved. Here, we employ a systematic metadynamics simulation based on environment-similarity collective variables that describe the local atomic environments of silicon to uncover the quartz-to-coesite reconstructive transition. Our results confirm that the preferential transformation from quartz $(10\overline{1}1)$ plane to coesite (010) plane in the previous experimental observation is driven by an energetically favored switching process. Large-scale simulations further reveal a synergistic direct-transformation and crystallization mechanism during the quartz-to-coesite transition, explaining the origin of coesite in shocked sandstones. These results elucidate the microscopic mechanisms of the quartz-to-coesite reconstructive transition, establish a transferable framework for studying reconstructive solid-state phase transformations, and provide new insight into ultrahigh-pressure metamorphic and hypervelocity impact processes.