Mineralogical and chemical properties of smectites in a sediment core from the southeastern Pacific

Abstract

A sediment core section nearly 80 cm long (50 to 125 cm below the sea bottom) from the southeastern Pacific is largely composed of smectites. X-ray, thermal, infrared, and chemical analyses indicate the smectites to be iron-rich montmorillonite (SiO 2, 50.17%; TiO 2, 0.77%; Al 2O 3, 7.11%; Fe 2O 3, 11.46%; FeO, 0.25%; MnO, 0.09%; MgO, 5.41%; CaO, 0.30%; Na 2O, 2.86%; K 2O, 0.57%; H 2O +, 8.02%; H 2O −, 12.03%; total, 99.04%) and aluminous montmorillonite (SiO 2, 49.39%; TiO 2, 1.18%; Al 2O 3, 14.39%; Fe 2O 3, 4.73%; FeO,—; MnO,—; MgO, 3.61%; CaO, 0.23%; Na 2O, 2.12%; K 2O, 0.34%; H 2O +, 6.86%; H 2O −, 15.20%; total, 98.05%). The iron-rich montmorillonite is rare and similar mineralogically and chemically to that of the northeastern Pacific previously reported by the authors. The aluminous montmorillonite is similar to the common type on land. The hypothesis that the iron-rich montmorillonite could have been formed by the interaction of hydrothermal solutions and seawater is supported by circumstantial evidence, whereas the aluminous montmorillonite was probably altered from volcanic ash transported from some other place.