SEM-EDX, Raman and infrared spectroscopic characterization of the phosphate mineral frondelite (Mn2+)(Fe3+)4(PO4)3(OH)5.

Abstract

We have analyzed a frondelite mineral sample from the Cigana mine, located in the municipality of Conselheiro Pena, a well-known pegmatite in Brazil. In the Cigana pegmatite, secondary phosphates, namely eosphorite, fairfieldite, fluorapatite, frondelite, gormanite, hureaulite, lithiophilite, reddingite and vivianite are common minerals in miarolitic cavities and in massive blocks after triphylite. The chemical formula was determined as (Mn0.68, Fe0.32)(Fe3+)3,72(PO4)3.17(OH)4.99. The structure of the mineral was assessed using vibrational spectroscopy. Bands attributed to the stretching and bending modes of PO3_ 4 and HOPO3_ 3 units were identified. The observation of multiple bands supports the concept of symmetry reduction of the phosphate anion in the frondelite structure. Sharp Raman and infrared bands at 3581 cm_1 is assigned to the OH stretching vibration. Broad Raman bands at 3063, 3529 and 3365 cm_1 are attributed to water stretching vibrational modes.