A first report on anion vacancies in a defect MgAl2O4 natural spinel

The chemical and structural features of a natural spinel sensu stricto (s.s.) sample were studied by a multi-analytical approach, including electron microprobe analysis (EMP), Fourier transform infrared spectroscopy (FTIR), and single crystal X-ray diffraction structural refinement (SREF). The sample, coming from impure dolomitic marbles of Pegu (Myanmar), has an anomalous chemistry with an Mg-content exceeding that of the ideal formula. In addition, a chemical zoning along a line-scan of EMP analyses was observed, with Mg and Al amounts showing opposite trends. The comparatively high and low concentrations, respectively, of divalent and trivalent cations lead to a deficit of positive charges. Thus, the requirement of neutrality of global charges for crystal structures appears to be violated, in this case. The possible reasons accounting for the anomalous chemistry are discussed. Based on the combined EMP, FTIR and SREF results, it is concluded that anion vacancies can adequately compensate for the observed deficit of positive charges. Thus, the analysed sample is a defect spinel. This is the first report of anion vacancies in a natural spinel s.s. With reference to the ideal formula MgAl2O4, the formation of anion vacancies, coupled to an excess of Mg and a deficiency of Al, may be described by the substitution mechanism 2Mg(2+)+(V)square -> 2Al(3+)+O2-, where (V)square represents an oxygen vacancy.