Ilvaite is sparsely disseminated in a serpentinized plagioclase wehrlite of the Asama igneous complex that underwent the Sambagawa regional metamorphism of pumpellyite-actinolite to greenschist facies. Ilvaite in the Asama complex is monoclinic (a = 13.019(5), b = 8.808(2), c = 5.850(4) Å, β = 90.19(4)°), and its composition is similar to the ideal end-member composition (CaFe22+Fe3+Si2O8OH). Ilvaite occurs in mats of serpentine (chrysotile); it probably formed during serpentinization, which might have accompanied the Sambagawa metamorphism. The associated secondary minerals include salitic clinopyroxene, magnetite and andradite. The ilvaite-free mineral assemblage that formed during the serpentinization is usually serpentine-clinopyroxene-magnetite, which is widespread in the complex. The phase relations between coexisting minerals suggest that the conditions during the formation of the ilvaite-bearing assemblage were reducing when compared to those of the assemblage serpentine-clinopyroxene-magnetite. The reducing conditions during the ilvaite formation were presumably brought about by hydrogen gas that1 was generated during the serpentinization of olivine.