In situ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries Crater, Germany

Reflected-light microscopy and fine-scale lasts microRaman spectroscopy of shocked garnet-cordierite-sillimanite gneisses in suevites of the Ries mete orite impact crater, Germany, led to the discovery of impact diamonds in th eir pristine graphite-diamond assemblages. Graphite-diamond textural relati ons permit a clear determination of the solid-state nature of the formation of diamond from graphite, which is estimated to have occurred at a peak-sh ock pressure between 30 and 40 GPa. Shock-induced transformations were prom oted only in unkinked and undeformed graphite booklets at the graphite-garn et, graphite-sillimanite, or graphite-rutile interfaces, where the differen ce in shock impedance is very high. Reverberations of shock waves with shor t wavelengths similar to the grain sizes at the phase boundaries are probab ly important constraints for dynamic graphite-diamond phase transformation. Raman spectroscopic investigations of hard transparent carbon platelets in tercalated between fine-grained diamond and deformed graphite revealed the platelets to be Raman inactive. The platelets are either dense amorphous ca rbon or an unknown dense crystalline carbon phase that is Raman inactive.