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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  and
Mamoru Adachi
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
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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.

Keywords

ilvaiteserpentinizationreducing conditionsAsama igneous complexMikabu greenstone beltSambagawa metamorphism
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 396 , September 1995 , pp. 489 - 496
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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