Spinel found in spherules from Cretaceous/Tertiary boundary sediments exhibits a wide range in composition and is distinguished from typical igneous spinel by high Mg, Al, and Ni, relatively low Ti and Cr, and high Fe2O3/FeO. Solid-solution compositions range from nearly pure magnetite to magnesioferrite to relatively pure MgAl2O4 spinel. The Ni-spinel trevorite is also a common component. A regional variation in composition is also observed: spinel from two North Pacific sites has higher Mg and Al and lower Ni and Fe than spinel from two European sites and one South Atlantic site. The most probable source of the spinel is crystallization from molten silicate droplets produced from a major impact event. The regional variations may provide a key to locating the impact site(s).