Chlorite and biotite weathering, Fe²⁺-rich corrensite formation, and Fe behavior under low P_O2 conditions and their implication for Precambrian weathering

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 P_O2 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 P_O2 conditions. The formation of Fe²⁺-rich corrensite and that of Fe²⁺-rich smectite or vermiculite in the laboratory at 1 atm of P_CO2 and ≤3 × 10⁻⁵ atm of P_O2 (Murakami et al. 2004) suggest that a possible Fe²⁺-bearing product during Precambrian weathering is Fe²⁺-rich sheet silicates but not siderite.