Detailed chemical and mineralogical analyses of authigenically altered impact spheroids contained in the Cretaceous/Tertiary (K/T) boundary clay of the northeastern Apennines provide information about the chemical conditions on the sea floor of a deep pelagic basin immediately after the impact of a large extraterrestrial object against the Earth's surface. The temporal alteration of global meterologic conditions resulting from this catastrophic event caused the death of most planktonic and nektonic biomass of the oceans. Dead organisms settled on the seafloor creating low-pH, low-Eh conditions at the water-sediment interface. Reducing and slightly acidic waters percolated in the top sediment, causing reduction and leaching of iron and dissolution of the top surface of the calcareous ooze. The suppression of biogenic calcium carbonate production in the upper water column, and the reducing conditions in the seafloor, lasted for a period not longer than 50 Ka. During this time, only detrital silt and clay were deposited in the basin, along with the fallout material derived from the impact cloud. The fallout included spherules of sub-millimeter size which condensed from vaporized and/or molten terrestrial target rock admixed with the vaporized impactor. The heterogeneous chemical composition of the spherules, and the wide range of shapes of relict crystals preserved in them, suggest that they were originally made of a suite of high-temperature quenched minerals ranging in composition from silica-rich/iron-poor ones (plagioclase), to iron-rich/silica-poor ones (spinel-beating clinopyroxene and olivine). The slightly reducing conditions of the seafloor favored the authigenic growth of K-feldspar in Si-rich spherules, and of glaucony in the Fe-rich ones. In some parts of the paleobasin, characterized by stronger reducing conditions, K-feldspar authigenesis did not occur, and spheroids were preferentially altered to pyrite and glaucony. The resumption of carbonate deposition resulting from the post-K/T plankton bloom coincided with the restoration of normal oxidizing conditions in most parts of the paleobasin. Such conditions caused the arrest of glauconitic growth and the alteration of the most Fe-rich spherules to goethite. Bioturbation activity also resumed in the top sediment, causing slight vertical mixing in the K/T boundary clay layer. In addition to the mineralogical and chemical description and the interpretation of the authigenic phases contained in the Italian K/T boundary clay, this paper includes a high-resolution stratigraphic analysis of the boundary clay in the Petriccio section which is the best preserved among the several K/T sections studied in the northeastern Apennines. The most relevant discovery from this study is that the impact spherules are concentrated in the central part of the clay layer along with the peak of an iridium anomaly, whereas grains of quartz with multisets of parallel lamellae, which are considered a product of impact shock-metamorphism, are mostly concentrated in the bottom part of the layer. This clear decoupling may represent several thousands of years of delay in deposition of iridium and spheroids with respect to shocked quartz and suggests two impact events closely spaced in time. However, further high-resolution analyses in other sections in the northeastern Apennines and elsewhere around the world is advised for more definitive confirmation for a terminal Cretaceous multiple-impact hypothesis.