The phosphate mineral sigloite Fe3+Al2(PO4)2(OH)3•7(H2O), an exception to the paragenesis rule - A vibrational spectroscopic study

The secondary phosphate mineral sigloite Fe3+Al2(PO4)(2)(OH)(3)center dot 7H(2)O is the exception to the rule that phosphate mineral paragenesis is related to the final phase of hydrothermal mineralization at tow temperatures. Sigloite was formed as an oxidation pseudomorph after paravauxite, during the last supergene paragenetic stage. We have studied the secondary phosphate mineral sigloite Fe3+Al2(PO4)(2)(OH)(3).7H(2)O using vibrational spectroscopic techniques. Because the mineral is a phosphate mineral, it is readily studied by spectroscopic techniques as the phosphate and hydrogen phosphate units are readily measured. Indeed, sigloite shows the presence of both phosphate and hydrogen phosphate units in its structure. Raman bands at 1009 cm(-1) with shoulders at 993 and 1039 cm(-1) are assigned to stretching vibrations of PO43- and HPO24- units. The Raman band at 993 cm(-1) is assigned to the nu(1) symmetric stretching mode of the POH units, whereas the Raman band at 1009 cm(-1) is assigned to the nu(1) PO43- symmetric stretching mode. Raman bands observed at 506, 528, 571, 596, 619 and 659 cm(-1) are attributed to the nu(4) out of plane bending modes of the PO4 and H2PO4 units. The Raman bands at 2988, 3118 and 3357 cm(-1) are assigned to water stretching vibration. The series of bands at 3422, 3449, 3493, 3552 and 3615 cm(-1) are assigned to the OH stretching vibrations of the hydroxyl units. The observation of multiple bands gives credence to the non-equivalence of the OH units in the sigloite structure. (C) 2012 Elsevier B.V. All rights reserved.