Magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron-rich portion of the Cretaceous-Tertiary (K-T) boundary clay at the Barranco del Gredero section, Caravaca, Spain. This spinel-type phase and others of the spinel group, found in K-T boundary clays at many widely separated sites, have been proposed as representing unaltered remnants of ejecta deposited from an earth-girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the Cretaceous period. The magnesioferrite occurs as euhedral, frequently skeletal, micron-sized octahedral crystals. The magnesioferrite contains 29 ?? 11 ppb Ir, which accounts for only part of the Ir anomaly at this K-T boundary layer (52 ?? 1 ppb Ir). Major element analyses of the magnesioferrite show variable compositions. Some minor solid solution exists toward hercynite-spinel and chromite-magnesiochromite. A trevorite-nichromite (NiFe2O4NiCr2O4) component is also present. The analyses are very similar to those reported for sites at Furlo and Petriccio, Umbria, Italy. On the basis of the morphology and general composition of the magnesioferrite grains, rapid crystallization at high temperature is indicated, most likely directly from a vapor phase and in an environment of moderate oxygen fugacity. Elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself, or from the mantle; there is no supporting evidence for its derivation from crustal target rocks. ?? 1986.