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

Here we used a coupling device between a scanning electron microscope (SEM) and a microRaman spectrometer to analyze micrometric aluminum-phosphate–sulfate (APS) occurring in a complex geological system. The capability to get BSE contrast, WDS elemental mapping and Raman data on tiny grains, allowed new structural insight for an APS with a high PO4/SO4 ratio and capability to distinguish compounds with complex variable compositions between the minerals alunite (KAl3(SO4)2(OH)6) and crandrallite (Ca(PO4)2(OH)5⋅H2O). [Display omitted] •APSs (aluminum-phosphate–sulfate) occur as complex micrometric grains.•A coupled SEM-microRaman spectrometer was used to differentiate APS phases.•Based on literature, band assignment of the Raman spectra was performed.•Sulfate substitution by two distinct phosphate groups could be evidenced.•Raman spectroscopy is proposed as a new tool to follow PO4/SO4 substitution. 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 1026cm−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.