Clay mineral analysis of deep-sea sediment cores raised from the Southern Ocean indicates that during the Oligocene and Eocene the sediments of less than two micron size contain an abundance of montmorillonite, 72 to 86%, 7 to 19% illite, and less than 5% kaolinite and chlorite combined. During the Miocene, a large decrease in montmorillonite abundance occurs, with concomitant increases in the other three major clay groups. In the Upper Miocene, a consistent abundance pattern with minor variations emerges for the last 5 m.y. of geologic time. Montmorillonite abundance ranges from 35 to 50%; illite, from 40 to 45%; and chlorite and kaolinite, 5 to 15% each. Measurements on the (001) basal reflection of illite indicate poor crystallinity characterizes illite occurring in the older sediments, Eocene through Miocene, while sediments deposited during the last 5 m.y. contain well crystallized illite. The silt-size fraction of sediments deposited during Eocene through Oligocene time contain an authigenic mineral assemblage with zeolites, barite and calcite, gradually replaced by detrital minerals during Miocene time. The silt-size fraction of sediments deposited during the last five million years consists of quartz, plagioclase, microcline, chlorite, muscovite and amphibole. The mineralogical changes observed during the Miocene have been interpreted as the dilution of the Eocene and Oligocene authigenic mineral assemblage by increased quantities of fine grained detrital minerals produced by intense weathering and erosion of the Antarctic continent caused by climatic changes. It is suggested the extensive glaciation that covered the Antarctic continent with massive ice sheets was probably initiated prior to the late Miocene.