Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low PO2 conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe2+-rich corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The <2 μm fraction of the weathered sample had an Fe2+/∑Fe value of 0.69, which, when combined with the presence of amorphous Fe3+ (hydr)oxides confirmed by TEM, indicates that the Fe2+/∑Fe value of corrensite is >0.69. These results indicate that on dissolution of chlorite and biotite, Fe2+ was transported as Fe2+ and precipitated as Fe2+-rich corrensite and a part of the dissolved Fe2+ was oxidized to amorphous Fe3+ (hydr)oxides under low PO2 conditions. The formation of Fe2+-rich corrensite and that of Fe2+-rich smectite or vermiculite in the laboratory at 1 atm of PCO2 and ≤3 × 10−5 atm of PO2 (Murakami et al. 2004) suggest that a possible Fe2+-bearing product during Precambrian weathering is Fe2+-rich sheet silicates but not siderite.

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