Abstract: We document the occurrence of Fe-bearing trioctahedral micas in the Poudrette quarry in the Mont Saint-Hilaire alkaline intrusion, characterize them by microprobe analysis, Mössbauer spectroscopy, X-ray diffraction, and optical measurements, describe their mineral chemistry, and discuss their petrological significance. In the nepheline and sodalite syenite, biotite and annite occur as coarse crystals characterized by low Al content (typically 2 atoms per formula unit, a.f.u.), high Mn content (typically 0.2 to 0.8 a.f.u.) and variable Fe/(Fe+Mg) values from 0.61 to 0.97. In the gabbro, biotite is less Fe-rich, has lower Mn content and high Ti content. Phlogopite is found as small metamorphic crystals in marble xenoliths within the syenite and siderophyllite occurs as large crystals in a metasomatized albitite dyke. Fe3+/Fetot values extend from 0.079 in the siderophyllite to 0.282 in a high-Fe3+ annite. All of the micas except for the phlogopite have high contents of (Fe3+)iv (∼0.13 to 0.45 a.f.u.) despite the high availability of Al in the rocks. We suggest that the high (Fe3+)iv amounts are caused by the high Mn abundance via a local structural mechanism. The great variety of mica encountered at Mont Saint-Hilaire reflects the highly heterogeneous conditions that prevailed during magmatic and postmagmatic crystallization in this intrusion.