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

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 (1011) 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.