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

Abstract

The secondary phosphate mineral sigloite Fe3+Al2(PO4)2(OH)3·7H2O is the exception to the rule that phosphate mineral paragenesis is related to the final phase of hydrothermal mineralization at low 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)3·7H2O 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 1009cm−1 with shoulders at 993 and 1039cm−1 are assigned to stretching vibrations of PO43- and HPO42- units. The Raman band at 993cm−1 is assigned to the ν1 symmetric stretching mode of the POH units, whereas the Raman band at 1009cm−1 is assigned to the ν1PO43- symmetric stretching mode. Raman bands observed at 506, 528, 571, 596, 619 and 659cm−1 are attributed to the ν4 out of plane bending modes of the PO4 and H2PO4 units. The Raman bands at 2988, 3118 and 3357cm−1 are assigned to water stretching vibration. The series of bands at 3422, 3449, 3493, 3552 and 3615cm−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.