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 + M3+ ↔ REE3+ + M2+ substitution since regions of higher Cr-concentration generally occur around chromite grains. The chromium content of epidote reaches 5.7 wt% Cr2O3 (0.36 Cr apfu/12.5 oxygens). In contrast, the Fe3+ content of the chromian epidote varies in a narrow range (5.1 to 8.9 wt% Fe2O3) 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 Fe3+-poor chromian epidote from other localities, whereas the Fe3+ content is greater. The substitution of Cr3+ 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.