Description
Zircons from six samples of the Coconino Formation, in the walls of Meteor Crater in Arizona, were compared with the largely hyacinth (pink--violet) zircon population from undisturbed Coconino sandstone; all zircons in all but one of the crater samples are colorless. Also, in samples of' sandstone from shafts in the crater fill, only a small percentage of zircons is hyacinth. Hyacinth zircon has previously been decolored by direct heating, over periods of several minutes or hours, and this process may be responsible for color loss in the samples from the crater. However, to determine a possible minimum time for such decoloration, two experiments were performed, involving high-brisance explosives and small-scale hypervelocity impact; both caused color loss. It is not known if the decoloration noted is due to passage of strong shock waves through the crystal lattice, or frictional heat developed during mega-and micro-brecciation. Impact-induced updraft and turbulent mixing of shattered rook, above the crater, heated and shocked to varying degrees, would account for the presence of colored zircons in the crater fill. More importantly, post-impact temperatures, at least in the uppermost crater fill, apparently were insufficient to decolor these zircons. The lowest decoloration temperature for the Coconino sandstone zircon population in and near the crater is about 300 degrees C based on heating periods of up to 36 hours. Results of this study indicate that color loss in hyacinth zircon may be used with such minerals as coesite and stishovite in evaluating suspected and supposed meteoritic impact sites.
