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Eleven samples (mostly of the Onaping Formation) from the 1.85-Ga Sudbury impact structure were examined with respect to their carbon phases. Six impact diamonds, <0.6 mm in diameter, were discovered in two samples of the Black Member of the Onaping Formation. These diamonds occur in a variety of colors, are cubic, and are generally friable due to inclusions and polycrystallinity. In one case, the hexagonal phase lonsdaleite was detected. According to transmission electron microscopy analyses, the mean grain size of individual diamond crystallites is in the range 50-100 nm. The diamonds are pervaded by numerous planar defects parallel to {111} and are organized in 100-200-nm-thick-bands, presumably inherited from precursor graphite. The diamonds were produced by the solid state transformation of graphite. Impact diamonds from the Onaping Formation are similar to other impact diamonds and are yet another expression of the effects of shock metamorphism at the Sudbury impact structure.

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