SEM, EDX and vibrational spectroscopy of the phosphate mineral vauxite from Llallagua, Bolívia

Abstract

We have undertaken a vibrational spectroscopic study of vauxite from Llallagua, Bolívia. This source is important source for rare and unusual secondary phosphate minerals and is the type locality for a number of rare phosphates such as vauxite, sigloite, metavauxite and for jeanbandyite. The chemical formula was determined as (Fe0.98Mn0.01)∑0.99(Al2.00)(PO4)∑2.03(OH)1.98·5.95(H2O).The Raman spectrum is dominated by intense Raman bands at 978, 1000, 1009, 1027cm−1 assigned to the PO43− and HPO42− stretching modes. Low intensity Raman bands are found at 1046, 1059, 1070, 1105, 1122, 1134 and 1150cm−1 and are assigned to the PO43− ν3 antisymmetric stretching vibrations. Raman bands of at 498, 502, 517, 523 and 535cm−1 are assigned to the ν4 PO43− bending modes while the Raman bands at 418, 451, 461 and 470cm−1 are due to the ν2 PO43− bending modes. The Raman spectral profile of vauxite in the hydroxyl stretching region is broad with component bands resolved at 2918, 3103, 3328, 3402, 3555 and 3648cm−1. Vibrational spectroscopy enables the assessment of the molecular structure of vauxite to be undertaken.