A thermogravimetric, scanning electron microscope and vibrational spectroscopic study of the phosphate mineral santabarbaraite from Santa Barbara mine, Tuscany, Italy

Abstract

A santabarbaraite mineral sample from Italy has been examined by thermal analysis, scanning electron microscopy with energy dispersive spectroscopy and vibrational spectroscopy. Chemical composition shows a Fe phosphate phase with minor amounts of Mn, Mg, K and Na were also observed. The santabarbaraite mineral was characterized by thermal analysis. Thermogravimetric analysis of the santabarbaraite mineral were obtained by using TA Instruments Inc. Q50 high-resolution TGA operating at a 10 °C min⁻¹ ramp with data sample interval of 0.50 s pt⁻¹ from room temperature to 1000 °C in a high-purity flowing nitrogen atmosphere (100 cm³ min⁻¹). Two mass loss steps are observed at 105 and 364.8 °C and are attributed to dehydration and dehydroxylation. Not all phosphate units are identical in the structure of santabarbaraite. This is reflected in the width of both the Raman and infrared bands. Two strong broad Raman bands observed at 1007 and 1095 cm⁻¹ are assigned to the phosphate ν ₁ symmetric and ν ₃ antisymmetric stretching mode. Raman bands observed at 561, 592 and 630 cm⁻¹ are assigned to the ν ₄ out of plane bending modes of the phosphate units. The observation of multiple bands supports the concept of non-equivalent phosphate units in the structure. Broad Raman bands observed at 3544 and 3611 cm⁻¹ are attributed to the OH stretching vibrations of the hydroxyl units. Vibrational spectroscopy enables subtle details of the molecular structure of santabarbaraite to be determined. Thermal analysis characterises the stability of the mineral.