Thermal analysis and Hot-stage Raman spectroscopy of the basic copper arsenate mineral

Abstract

The thermal analysis of euchroite shows two mass loss steps in the temperature range 100–105 °C and 185–205 °C. These mass loss steps are attributed to dehydration and dehydroxylation of the mineral. Hot-stage Raman spectroscopy (HSRS) has been used to study the thermal stability of the mineral euchroite, a mineral involved in a complex set of equilibria between the copper hydroxy arsenates: euchroite Cu2(AsO4)(OH)·3H2O → olivenite Cu2(AsO4)(OH) → strashimirite Cu8(AsO4)4(OH)4·5H2O → arhbarite Cu2Mg(AsO4)(OH)3. HSRS inolves the collection of Raman spectra as a function of the temperature. HSRS shows that the mineral euchroite decomposes between 125 and 175 °C with the loss of water. At 125 °C, Raman bands are observed at 858 cm−1 assigned to the ν1 AsO43− symmetric stretching vibration and 801, 822, and 871 cm−1 assigned to the ν3 AsO43− (A1) antisymmetric stretching vibrations. A distinct band shift is observed upon heating to 275 °C. At 275 °C, the four Raman bands are resolved at 762, 810, 837, and 862 cm−1. Further heating results in the diminution of the intensity in the Raman spectra, and this is attributed to sublimation of the arsenate mineral. HSRS is the most useful technique for studying the thermal stability of minerals, especially when only very small amounts of mineral are available.