Vibrational spectroscopic characterization of the phosphate mineral ludlamite (Fe,Mn,Mg)3(PO4)2⋅4H2O – A mineral found in lithium bearing pegmatites

Abstract

The objective of this work is to analyze ludlamite (Fe,Mn,Mg)3(PO4)2⋅4H2O from Boa Vista mine, Galiléia, Brazil and to assess the molecular structure of the mineral. The phosphate mineral ludlamite has been characterized by EMP-WDS, Raman and infrared spectroscopic measurements. The mineral is shown to be a ferrous phosphate with some minor substitution of Mg and Mn.Raman bands at 917 and 950cm−1 are assigned to the symmetric stretching mode of HOPO32- and PO43- units. Raman bands at 548, 564, 599 and 634cm−1 are assigned to the ν4PO43- bending modes. Raman bands at 2605, 2730, 2896 and 3190cm−1 and infrared bands at 2623, 2838, 3136 and 3185cm−1 are attributed to water stretching vibrations. By using a Libowitzky empirical function, hydrogen bond distances are calculated from the OH stretching wavenumbers. Strong hydrogen bonds in the structure of ludlamite are observed as determined by their hydrogen bond distances. The application of infrared and Raman spectroscopy to the study of ludlamite enables the molecular structure of the pegmatite mineral ludlamite to be assessed.