Melanite from the endoskarn of a spessartite dyke intruding into the Kinshozan limestone in Ogaki City, Gifu Prefecture

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A spessartite dyke that intruded into the Permian Kinshozan limestone formed endoskarn that is rich in melanite, along with clinopyroxene and apatite, in a matrix of secondary calcite, thomsonite, stilbite, heulandite and apophyllite. Garnet grains, ranging in size from 0.5 to 3 mm, have dark brown cores and pale to colorless mantles. Detailed analysis of a typical grain reveals TiO2 contents from 2.70 to 11.90 wt.% in the core and from 0.42 to 2.25 wt.% in the mantle. Cation proportions can be attributed to a melanite chemistry controlled by two kinds of substitutions, Ti^4+(Z)=Si(Z) and Fe^2+(Y)Ti^4+(Y)=2^M3+(Y). Iron occurs in both ferric and ferrous states, with Ti substitutions negatively correlated with intermediate to high Fe^3+/(Fe^3++Al). Decreasing ferrous iron from core to mantle is consistent with reverse Mg/(Mg+Fe^2+) zoning in co-genetic clinopyroxene. Low fO2, conditions probably enhanced Fe^2+(Y)Ti^4+(Y)=2M^3+(Y) substitution in garnet.

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