Crystal chemistry of two detrital Cr-spinels with unusually low values of oxygen positional parameter: oxidation mechanisms and possible origin

Two natural chromian spinels from the 'terra rossa' deposits in the Trieste Karst (NE Italy) were studied by means of single-crystal X-ray diffraction and microprobe chemical analysis. Crystallographic and chemical data yield ed information about the mechanisms and degrees of oxidation undergone by t he samples in a sedimentary environment. Therefore, mineral formulae were r ecalculated according to a two-stage mechanism, already tested in artificia lly oxidized spinels, and cations and vacancies were partitioned. The site cation distribution was found to agree with the cation distribution which t he probably parent stoichiometric sample would have reached if it had evolv ed in an oxidizing environment.