Distribution and origin of impact diamonds in the Ries crater, Germany
R.T. Schmitt, C. Lapke, C.M. Lingemann, M. Siebenschock, D. Stöffler, 2005. "Distribution and origin of impact diamonds in the Ries crater, Germany", Large Meteorite Impacts III, Thomas Kenkmann, Friedrich Hörz, Alex Deutsch
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The distribution and origin of impact diamonds in the ejecta blanket of the Ries crater, Germany, was investigated. Impact diamonds are present in the fallout suevite, whereas the cataclastic crystalline breccias, lithic impact breccia (Bunte Breccia) and clast-rich impact melt rock do not contain diamonds. No regional concentrations of impact diamonds in the fallout suevite could be detected. The average concentration of diamonds is ∼0.1–0.2 ppm. The carriers of impact diamonds are specific suevite components, such as graphite-bearing crystalline rock fragments of shock stage III, and most likely small fragments thereof in melt fragments and suevite matrix. Impact diamonds occur as pseudohexagonal, transparent, and birefringent plates, which reach sizes up to 300 µm. Their color is commonly greenish, but can also be black, gray, yellow, or colorless. Most of the impact diamonds have a fibrous or spongy internal structure and extensional microfractures, which lead to a characteristic porosity. This is the result of a volume decrease due to the phase transformation of graphite to diamond. The Raman characteristics of these impact diamonds are discussed in detail. The strong morphologic similarity of impact diamonds to the precursor graphite from the crystalline target rocks indicate a solid state martensitic phase transformation which occurs at shock pressures of 45–55 GPa.