Coupled SEM-microRaman system: A powerful tool to characterize a micrometric aluminum-phosphate–sulfate (APS)

A micrometric aluminum-phosphate–sulfate (APS) with an intermediate composition between the minerals woodhouseite (CaAl3(PO4)(SO4)(OH)6) and crandallite (Ca(PO4)2(OH)5⋅H2O) has been identified in a natural sample and characterized using a coupled SEM-microRaman system. The Raman spectrum obtained was characterized by various bands assigned to vibrational modes of sulfate and phosphate units. A comparison with other APS minerals, with different PO4/SO4 ratios, enables some differences to be highlighted, which concern particularly the intensities of some sulfate and phosphate bands. It is established that the intensity of the sulfate bands located at around 653 and 1026 cm−1 decrease as a function of the mole fraction of phosphate in the APS minerals. This correlation reflects well the effect of the substitution of sulfate anions by phosphate anions. In addition, this study has also shown that the use of a coupling system between a SEM and a microRaman spectrometer is a powerful tool to identify micro-minerals of the alunite supergroup occurring in a complex geological sample.