Nb-rich brookite from Gross Brukkaros, Namibia: substitution mechanims and Fe2+/Fe3+ ratios

Abstract

<jats:title>Abstract</jats:title><jats:p>Idioblastic, mm-scale brookite is reported from SiO<jats:sub>2</jats:sub>-Ba-Fe-rich metasomatic rocks from the carbonatitic Gross Brukkaros Volcanic Field, Southern Namibia. Electron probe microanalysis shows significant concentrations of Nb (12–17 wt.% Nb<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub>), coupled with Fe, substituting for Ti in the structure. Calculation of the analytical data indicates evolving Fe<jats:sup>2+</jats:sup>/(Fe<jats:sup>2+</jats:sup>+Fe<jats:sup>3+</jats:sup>) ratios during crystallization and two distinct substitution mechanisms in operation: 2Ti<jats:sup>4+</jats:sup> ⇋ Nb<jats:sup>5+</jats:sup> + Fe<jats:sup>3+</jats:sup> and 3Ti<jats:sup>4+</jats:sup> ⇋ 2Nb<jats:sup>5+</jats:sup> + Fe<jats:sup>2+</jats:sup>. The compositional patterns observed are explained in terms of initial growth in an environment which evolved from oxidizing to reducing. The outermost 10 µm rims of the crystals are characterized by oxidizing conditions, resulting from a subsequent metasomatic overprint.</jats:p>