Nomenclature of amphiboles Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names

Chromian epidote was found in pebbles of omphacite rock derived from the Sambagawa metamorphic rocks in central Shikoku, Japan. The pebbles consist of chromian epidote, omphacite, amphibole, muscovite, phlogopite, chromite, albite, and zircon. The chromian epidote crystals are dark yellow to brown. Microscopically, they are subhedral and are pleochroic from yellowish orange to pale yellow colorless. Chromian epidote has a zonal structure. Typically, cores of Sr rich epidote are overgrown with Ca epidote; alternatively Ca epidotes are rimmed by and/or intergrown with REE rich epidote. However, Cr distribution is not related to the zonal structure caused by Ca ↔ Sr and Ca + M 3+ ↔ REE 3+ + M 2+ substitution since regions of higher Cr concentration generally occur around chromite grains. The chromium content of epidote reaches 5.7 wt% Cr 2 O 3 (0.36 Cr apfu/12.5 oxygens). In contrast, the Fe 3+ content of the chromian epidote varies in a narrow range (5.1 to 8.9 wt% Fe 2 O 3 ) irrespective of the chromium content. Associated minerals surrounding chromite (omphacite, amphibole, muscovite, and phlogopite) also tend to have a high and variable chromium content caused by Al ↔ Cr substitution, but with a nearly constant Fe content in individual minerals. Chromite is considered to be the source material of the chromian epidote and the associated minerals. The heterogeneous distribution of chromium may be attributed to the immobility of chromium under metamorphic conditions. The maximum Cr content of the chromian epidote in the present study is less than that of Fe 3+ poor chromian epidote from other localities, whereas the Fe 3+ content is greater. The substitution of Cr 3+ for Al in the M3 site of the studied chromian epidote may be limited by the ferric iron occupying the M3 site to the extent of 0.52 apfu.

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“…Nomenclature used for pyroxene and amphibole is after Morimoto et al (1988), and Leake et al (1997).…”

Section: Associated Mineralsmentioning

confidence: 99%

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

Nagashima

Akasaka

Sakurai

2006

Journal of Mineralogical and Petrological Sciences

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“…2b and 2c). Amphibole crystals in the matrix are chemically homogeneous or slightly zoned with a rimward decrease in the A -site occupancy, and plot mostly within the magnesiohornblende and tschermakite fields according to Leake et al (1997) (Fig. 3b).…”

Section: Amphibolementioning

confidence: 99%

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Endo

Nowak

Wallis

2013

Journal of Mineralogical and Petrological Sciences

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Quantitative analyses of garnet -bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high -grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn -tectonic prograde -zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P) -temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H 2 O) give consistent peak -P estimates of 1.3 -1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ~ 0.7 GPa, which is as high as the values reported in well -characterized eclogite samples in central Shikoku. The inferred peak -P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ~ 0.8 GPa, 580 °C suggest decompression to the epidote -amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re -equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

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“…The amphibole nomenclature used in this paper is that proposed by Leake et al (1997) § Number of analytical points. # Chemical composition used for obtaining the reaction curves in Figure 5.…”

Section: Coarse-grained Zoisite Corundum Garnet and Amphibolementioning

confidence: 99%

Mg-rich chloritoid in a corundum-bearing zoisite rock from the Sanbagawa belt, central Shikoku, Japan

Banno

2009

Journal of Mineralogical and Petrological Sciences

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Mg -rich chloritoid with the ratio Mg/(Mg + Fe 2+ ) = 0.37 to 0.49 was found in a corundum -bearing zoisite rock from the Besshi area in the Sanbagawa metamorphic belt, Japan. The zoisite rock consists mainly of zoisite, corundum, garnet, and amphibole. Corundum was retrogressively replaced by secondary chloritoid, zoisite, clinozoisite, chlorite, and paragonite. Staurolite rarely occurs in aggregates of the secondary phases (chloritoid, chlorite, and paragonite), which occur along the cracks in corundum, and is surrounded by chloritoid. This fact indicates that chloritoid was formed by the following retrogressive reaction: 7chlorite + 8corundum + 4staruol-ite + 15H 2 O →51chloritoid. The estimated temperature for the above univariant reaction in the MASH system is 637 ± 30 °C at 10 kbar and 744 ± 34 °C at 20 kbar. The textural evidence and the estimated temperature suggest that chloritoid formed during the exhumation stage from eclogite to epidote -amphibolite facies conditions.

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Nomenclature of amphiboles Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names

Cited by 1,054 publications

References 12 publications

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Mg-rich chloritoid in a corundum-bearing zoisite rock from the Sanbagawa belt, central Shikoku, Japan

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