The molecular structure of the phosphate mineral beraunite Fe^2+Fe5^3+(PO4)4(OH)5.4H2O – A vibrational spectroscopic study.

Abstract

The mineral beraunite from Boca Rica pegmatite in Minas Gerais with theoretical formula Fe^2+ Fe5^ 3+(PO4)4(OH)5.4H2O has been studied using a combination of electron microscopy with EDX and vibrational spectroscopic techniques. Raman spectroscopy identifies an intense band at 990 cm^-1 and 1011 cm^-1. These bands are attributed to the PO4^-3 v1 symmetric stretching mode. The v3 antisymmetric stretching modes are observed by a large number of Raman bands. The Raman bands at 1034, 1051, 1058, 1069 and 1084 together with the Raman bands at 1098, 1116, 1133, 1155 and 1174 cm^-1 are assigned to the v3 antisymmetric stretching vibrations of PO4^-3 and the HOPO3^-2 units. The observation of these multiple Raman bands in the symmetric and antisymmetric stretching region gives credence to the concept that both phosphate and hydrogen phosphate units exist in the structure of beraunite. The series of Raman bands at 567, 582, 601, 644, 661, 673, and 687 cm^-1 are assigned to the PO4^-3 v2 bending modes. The series of Raman bands at 437, 468, 478, 491, 503 cm^-1 are attributed to the PO4^-3 and HOPO3^-2 v4 bending modes. No Raman bands of beraunite which could be attributed to the hydroxyl stretching unit were observed. Infrared bands at 3511 and 3359 cm^-1 are ascribed to the OH stretching vibration of the OH units. Very broad bands at 3022 and 3299 cm^-1 are attributed to the OH stretching vibrations of water. Vibrational spectroscopy offers insights into the molecular structure of the phosphate mineral beraunite.