Disordered chromite in the Martian meteorite Allan Hills 84001

At room temperature the Fe-57 Mossbauer spectrum of chromite particles separated from the Martian meteorite Allan Hills 84001 is different from the Mossbauer spectrum of natural chromite. One hypothesis for this discrepancy is that the chromite in Allan Hills 84001 is found in a disordered or even amorphous state, possibly caused by a shock event. As a test of this hypothesis, we have attempted to amorphize natural chromite by ion implantation. Fe-57 emission Mossbauer spectra of chromite implanted with radioactive precursor Mn-57 ions have been measured. The spectra can be analyzed in terms of two contributions, a crystalline chromite fraction and a disordered fraction, attributed to disordered iron-chromium oxide or amorphous zones created by the implantation process. The Mossbauer parameters of the amorphous fraction are indistinguishable from those deduced from the spectrum of the chromite separate from Allan Hills 84001. A shock event is a likely explanation for these findings and with appropriate energy calibration this may possibly be used to estimate the intensity of the shock event that once modified minerals in the meteorite Allan Hills 84001.