Formation of Mg-rich kuliginite (Fe3Mg(OH)6Cl2) during serpentinization by saline fluids

Serpentinization generally occurs in the presence of Cl-bearing fluids, but formation of chloride minerals and their role in serpentinization are poorly understood. Kuliginite, a hydroxychloride mineral with the ideal formula of Fe₃Mg(OH)₆Cl₂, was recently identified in serpentinized rocks from New Caledonia, and it was suggested that kuliginite may significantly influence iron distribution and hydrogen generation during serpentinization by Cl-rich fluids. In this study, we re-investigate kuliginite from New Caledonia, and reveal that the kuliginite is mostly Mg-rich and Fe-depleted compared with holotype kuliginite, in contrast to previous conclusion that the kuliginite closely resembles holotype kuliginite. Overall, Mg-rich kuliginite formed preferentially over holotype-like kuliginite in serpentinized rocks from New Caledonia, which may be ascribed to higher stability of the former than the latter. Despite some substitution of Fe by Mg, Mg-rich kuliginite still contains large amounts of iron (∼ 50 wt% FeO). Therefore, Mg-rich kuliginite can also greatly impact iron partitioning and hydrogen generation once it forms during serpentinization by Cl-rich saline fluids.