The moissanite enigma: a new finding in syenite xenoliths from the Azores Islands

Our discovery of moissanite grains in peralkaline syenite from the Água de Pau Volcano (São Miguel, Azores Islands) represents the first report of this mineral in an oceanic geodynamic settings. Raman spectroscopy and single-crystal X-ray diffraction indicate the predominance of the 6H over the 4H hexagonal polytypes. Trace elements distribution is homogeneous except for Al and V. Azorean moissanite often host rounded inclusions of metallic Si and other still unknown metallic alloys. A process involving a flushing of CH4-H2 ultra-reducing fluids in the alkaline melts might be considered as the possible origin of natural SiC thus calling for local strongly reducing conditions for the mantle beneath the São Miguel Island.