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

Abstract

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 27-38 - DOI: 10.2451/2011PM0003
Special Issue in memory of Sergio Lucchesi

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

Rosa Anna Fregola^1,*, Ferdinando Bosi² and Henrik Skogby<sup>3

1</sup>Dipartimento Geomineralogico, Università degli Studi di Bari "Aldo Moro", Italy
²Dipartimento di Scienze della Terra, Sapienza Università di Roma, Italy
³Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
*Corresponding author: r.fregola@geomin.uniba.it

Abstract

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 2Mg²⁺+^V□→2Al³⁺+O^2-, where ^V□ represents an oxygen vacancy.


Key words: natural spinel; exceeding Mg-content; chemical zoning; anion vacancies.