The molecular structure of the phosphate mineral beraunite Fe2+Fe53+(PO4)4(OH)5⋅4H2O – A vibrational spectroscopic study

Abstract

The mineral beraunite from Boca Rica pegmatite in Minas Gerais with theoretical formula Fe2+Fe53+(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 990cm−1 and 1011cm−1. These bands are attributed to the PO43− ν1 symmetric stretching mode. The ν3 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 1174cm−1 are assigned to the ν3 antisymmetric stretching vibrations of PO43− and the HOPO32− 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 687cm−1 are assigned to the PO43− ν2 bending modes. The series of Raman bands at 437, 468, 478, 491, 503cm−1 are attributed to the PO43− and HOPO32− ν4 bending modes.No Raman bands of beraunite which could be attributed to the hydroxyl stretching unit were observed. Infrared bands at 3511 and 3359cm−1 are ascribed to the OH stretching vibration of the OH units. Very broad bands at 3022 and 3299cm−1 are attributed to the OH stretching vibrations of water. Vibrational spectroscopy offers insights into the molecular structure of the phosphate mineral beraunite.