The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, porcellanites and cherts in land sections and Deep Sea Drilling Project and Ocean Drilling Program drill sites are usually found in sediment sections of Miocene age and older. During R.V. Polarstem cruises ANT-IX/3 and 4, young porcellanites were recovered for the first time in contact with their host sediment in two cores from the Atlantic sector of the southern ocean. Chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation. In both cores the porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid transformation of biogenic silica into porcellanites.