A New Mechanism for Stishovite Formation During Rapid Compression of Quartz and Implications for Asteroid Impacts

Shock‐induced transformations of quartz to high‐pressure polymorphs and diaplectic glass are decisive in identifying impact cratering events. Under shock compression, quartz can melt in local hot spots and crystallization of these silica melts under pressure can yield the high‐pressure mineral stishovite. A solid‐state transition to stishovite in relation to the formation of amorphous lamellae was already suggested in the late 1960s, but this idea was never comprehensively proven. Therefore, the mechanism responsible for such an intracrystalline stishovite formation is unknown to date. Herein, crystallographically oriented single crystals of quartz were compressed and decompressed in a membrane‐driven diamond anvil cell. These experiments aim at simulating the pressure paths of natural impacts on the timescale of seconds using compression rates between 0.2 and 0.6 GPa/s and peak pressures between 20 and 37 GPa. During the compression of quartz, the time‐resolved synchrotron X‐ray diffraction patterns reveal the almost simultaneous formation of two high‐pressure polymorphs, the recently identified rosiaite‐structured silica and stishovite. Transmission electron microscopic observations of recovered samples show that stishovite occurs as arrays of uniformly oriented nanometer‐sized crystals in amorphous intracrystalline lamellae. These observations indicate that the numerous stishovite crystals likely nucleated from the structurally similar rosiaite phase and thus inherited their uniform orientation during compression. During decompression, the metastable and non‐quenchable rosiaite‐structured phase collapsed to the amorphous stishovite‐containing lamellae. These findings attest to a novel mechanism of the formation of stishovite in the solid state and provide an explanation for similar microstructural occurrences of stishovite in impact‐metamorphic rocks and shocked meteorites. The extreme pressures and temperatures of asteroid impacts on planetary surfaces result in irreversible changes in the affected rocks. The mineral quartz can then display characteristic amorphous lamellae and crystals of the high‐pressure mineral stishovite. While stishovite usually crystallizes from locally molten silica under pressure, its formation in the solid state free of silica melts has remained enigmatic to date. Here, we simulated asteroid impacts on the time scale of seconds and at ambient temperature through experiments of rapid compression and decompression of quartz. The crystallographic changes during the experiments and the microstructural observations of the recovered samples provide evidence that stishovite is indeed formed in the solid state during compression of quartz. In particular, a large number of stishovite crystals form in an identical crystallographic orientation through nucleation from the structurally similar rosiaite‐structured silica phase. The latter is formed metastably during compression. The final microstructure consists of the numerous stishovite crystals located in amorphous lamellae, because the metastable rosiaite‐structured phase transforms into amorphous silica during decompression. The discoveries describe a novel mechanism for rapid stishovite formation in the solid state, explaining very similar stishovite occurrences in meteorites and rock samples recovered from impact sites. Stishovite can form in the solid state from compressed quartz through nucleation from a transient rosiaite‐structured silica phaseThe solid state transition from quartz to finally stishovite results in a uniform orientation relationshipShock‐metamorphic features in quartz can be mimicked by compression in the diamond anvil cell Stishovite can form in the solid state from compressed quartz through nucleation from a transient rosiaite‐structured silica phase The solid state transition from quartz to finally stishovite results in a uniform orientation relationship Shock‐metamorphic features in quartz can be mimicked by compression in the diamond anvil cell