A vibrational spectroscopic study of philipsbornite PbAl3(AsO4)2(OH)5 H2O - molecular structural implications and relationship to the crandallite subgroup arsenates.

Abstract

The presence of arsenic in the environment is a hazard. The accumulation of arsenate by a range of cátions in the formation of minerals provides a mechanism for the remediation of arsenate contamination. The formation of the crandallite group of minerals provides a mechanism for arsenate accumulation. Among the crandallite minerals are philipsbornite, arsenocrandallite and arsenogoyazite. Raman spectroscopy complimented with infrared spectroscopy has enabled aspects of the structure of philipsbornite to be studied. The Raman spectrum of philipsbornite displays an intense band at around 840 cm_1 attributed to the overlap of the symmetric and antisymmetric stretching modes. Raman bands observed at 325, 336, 347, 357, 376 and 399 cm_1 are assigned to the m2 (AsO4)3_ symmetric bending vibration (E) and to the m4 bending vibration (F2). The observation of multiple bending modes supports the concept of a reduction in symmetry of the arsenate anion in philipsbornite. Evidence for phosphate in the mineral is provided. By using an empirical formula, hydrogen bond distances for the OH units in philipsbornite of 2.8648 Å, 2.7864 Å, 2.6896 Å cm_1 and 2.6220 were calculated.