Mixed-Layer Chlorite-Corrensite Minerals in the Hydrothermally Altered Upper Pleistocene Sediments, ODP Hole 1036A, Juan de Fuca Ridge, Pacific Ocean

The article provides detailed structural and crystal-chemical characteristics of mixed-layer chlorite-corrensite minerals in <1-μm fractions extracted from Upper Pleistocene sediment samples from ODP Hole 1036A, which was drilled at the Dead Dog hydrothermal field in the northern Middle Valley, Juan de Fuca Ridge, in the northeastern Pacific Ocean. Using the XRD pattern modeling method, it was shown for the first time that the studied mixed-layer minerals should be considered as chlorite-corrensites rather than chlorite-smectites. In the structure of the studied mixed-layer minerals, trioctahedral chlorite and corrensite layers occur in a ratio of 50 : 50–60 : 40, and ~90 : 10, alternating with a tendency to segregation at a short-range order factor R = 1. Averaged crystal-chemical formulas were obtained for the mixed-layer chlorite-corrensites and their individual chlorite and corrensite layers. It is assumed that the model with an asymmetric distribution of charges in tetrahedral sheets of 2 : 1 layers is realized in corrensite packages of natural chlorite-corrensite. All samples belong to Mg–Fe varieties of chlorite-corrensites. The most probable mechanism for the formation of chlorite-corrensites of different compositions is the dissolution of original terrigenous clay minerals during the interaction of hydrothermal fluid with terrigenous sediments and the synthesis of these mixed-layer minerals from solution.