Alteration of metamict, rare-earth, AB 2O 6-type NbTaTi oxides

Abstract

Primary and secondary alteration affect composition, physical properties and the results of annealing studies of metamict, rare-earth, AB 2O 6-type Nb-Ta-Ti oxides. Primary alteration (e.g. hydrothermal) causes a consistent increase in calcium content, generally a decrease in the uranium and thorium content, a decrease in the absolute rare earth concentrations, (with only one exception, the REE distributions remain constant), a slight decrease in B-site cations, and an increase in structural and absorbed water. Secondary alteration (e.g. weathering) is similar but produces a decrease in Ca content, an increased leaching of A-site cations and relative increase in B-site cations. R.I., S.G., and reflectance (546 nm) decrease with both types of alteration, but VHN 50 remains approximately constant in altered and unaltered areas of the same specimen. The deviation from stoichiometric proportions of A- and B-site cations with increased alteration results in complex recrystallization phase assemblages during annealing studies (500–1000°C at 1 bar). The effect of alteration on systems of nomenclature and identification procedures is discussed.