Mineralogy and Geochemistry of Aluminum Phosphates from the Montebras Pegmatite, Massif Central, France

The Montebras pegmatitic cupola, Massif Central, France, hosts large pods of aluminum phosphates mainly composed of primary amblygonite–montebrasite. These phosphate masses are considerably altered by post-magmatic processes, leading to the formation of a number of secondary metasomatic hydrothermal phosphates (lacroixite, wardite, morinite, viitaniemiite, fluorapatite, triplite, eosphorite) and low-temperature phosphates (fluorapatite, crandallite, goyazite, gorceixite, variscite, turquoise, wavellite). A Mn-rich viitaniemiite-like phase, as well as an unknown Na-Al-P fluoride, were also identified. These phosphate assemblages were first observed with a classical polarizing microscope, but we demonstrated that cathodoluminescence microscopy was a powerful tool to decipher the complex petrography of these phases. Electron-microprobe analyses, as well as laser ablation inductively coupled plasma-mass spectroscopy, were used to characterize the trace- and major-elemental composition of those phosphates. These results allowed the determination of genetic sequences, explaining the crystallization of aluminum phosphates at different stages of the Montebras pegmatite evolution.