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Ilvaite from a serpentinized peridotite in the Asama igneous complex, Mikabu greenstone belt, Sambagawa metamorphic terrain, central Japan

Published online by Cambridge University Press:  05 July 2018

Takashi Agata
Affiliation:
Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-01 Japan
Mamoru Adachi
Affiliation:
Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-01 Japan

Abstract

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 (). 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.

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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