Raman spectroscopic analysis and in-situ heating experiments on adranosite minerals

Abstract

This study presents a comprehensive analysis of the rare ammonium sulfates, adranosite (NH4)4 NaAl2(SO4)4Cl(OH)2 and adranosite-(Fe) (NH4)4NaFe(3+)2(SO4)4Cl(OH)2, found in volcanic fumarole and burning coal heap environments. Using Raman spectroscopy, this research provides the first detailed characterization of samples from the type locality of La Fossa, Italy, as well as from several coal-fire sites in Central Europe. The investigation identified key spectral differences between the aluminum (adranosite) and iron (adranosite-(Fe)) end-members, particularly in the vibrational band positions for sulfate, hydroxyl, and metal-oxygen groups. For specimens of adranosite with additional iron content (iron-rich adranosite), a splitting of specific spectral bands was observed, directly reflecting the chemical substitutions in the structure. Furthermore, thermal stability was examined using thermo-Raman spectroscopy. The results show that iron-rich adranosite is stable up to approximately 180-200 °C. Above this temperature, the mineral begins to decompose, releasing volatile components like ammonium and hydroxyl groups. The final crystalline product, formed between 460 and 500 °C, was identified as the anhydrous phase Na3Al(SO4)3 corresponding to the mineral heimaeyite. These findings provide reference data for the accurate identification of adranosite-group minerals and enhance the understanding of their structural behavior and stability under the high-temperature conditions commonly found in fumaroles and coal fires.