I've read about chemical aspects of sector zoning in zoisite (variations in
Fe3/Fe3+Al ratios: Deer, Howie & Zussman, 1986)... Going further, does
anybody have any opinion about the origin(s) of this rare feature?
I've found sector-zoned zoisite porphyroclasts in poorly deformed eclogite.
Zoisite coexists with omphacite porphyroclasts and garnet, which is
arranged in honeycomb patterns (surrounding omphacite and zoisite
It seems therefore that eclogitization took place in this rock at nearly
static conditions, with low-range chemical diffusion... Is then zoisite
reflecting some chemical inhomogeneity in a pristine (now diappeared)
phase? ... I'm thinking about possible An-rich, (zoned?) igneous
PS:... Trace amounts of trivalent cations in the former phase can also
account for tiny rutile inclusions, arranged in preferred orientations
within zoisite clasts.
Dpto. Mineralogía y Petrología
Facultad de Ciencias, Univ. del País Vasco
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E-48080 Bilbao, Spain
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