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

Abstract

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.

Distribution of chromium among the octahedral sites in chromian epidote from Iratsu, central Shikoku, Japan

“…Nagashima et al (2006) distinguished three types of chromian epidote: Ca epidote with < 3 wt% SrO, Sr rich epidote with 3 11 wt% SrO, and the rare earth elements (REE) rich epidote with 2 7 wt% (Ce 2 O 3 + La 2 O 3 + Nd 2 O 3 + Pr 2 O 3 + Sm 2 O 3 + Gd 2 O 3 ), and described the cores of the Sr rich epidote being overgrown with Ca epidote, whereas the Ca epidotes were rimmed by and/or intergrown with REE rich epidotes. The Cr 2 O 3 content in the epidote is variable and attains up to 6 wt% around the chromite grains.…”

“…Recently, Nagashima et al (2006) reported on the occurrence of chromian epidote containing up to 6 wt% Cr 2 O 3 in pebbles of omphacite rock from the Sambagawa metamorphic rocks in central Shikoku, Japan. In the present study, we have investigated the structure of chromian epidote obtained using single crystal X ray diffraction coupled with 57 Fe Mössbauer spectroscopy and optical spectroscopy.…”

“…The Cr distribution is not related to any zonal structure. The Fe 3+ content varies within a narrow range: 5 7 or 6 9 wt% of the total Fe 2 O 3 (Nagashima et al, 2006).…”

“…The details of the analytical conditions of the EPMA analysis have been described in Nagashima et al (2006). Finally, three chromian epidote grains with relatively high Cr 2 O 3 content were selected for X ray data collection.…”

“…The Cr distribution is not related to any zonal structure. The Fe 3+ content varies within a narrow range: 5 7 or 6 9 wt% of the total Fe 2 O 3 (Nagashima et al, 2006).In order to analyze the oxidation state of the iron and chromium using 57 Fe Mössbauer spectroscopy and optical spectroscopy, respectively, the chromian epidote was separated from the omphacite rock pebbles using the following procedure. The pebbles were manually crushed down to grains 0.25 0.5 mm in diameter, and the chromian epidote grains were then separated using a Franz isodynamic separator.…”