SPINEL FORMATION IN THE MESOSPHERE OF THE EARTH AFTER CRETACEOUS/TERTIARY IMPACT. S. Aslanian, A. Preisinger and L. Petrás, Institute of Mineralogy, Crystallography and Structural Chemistry, Tech. University of Vienna, A-1060 Vienna, Getreidemarkt 9, AUSTRIA.

Magnetic spinels separated from hemipelagic and pelagic sediments of the Cretaceous/Tertiary (K/T) boundary sections of Bjala, Bulgaria and Cerbara, Italy were analysed by X-ray powder diffraction and by a scanning electron microscope equipped with microprobe.

65 million years ago Bjala was located at a distance of about 10000 km and Cerbara about 8800 km from the impact structure in Yucatan, Mexico. It is proposed, that the spinels found in the K/T boundaries are formed as re-condensed material of a hot fire ball induced by the impact in the mesosphere of the Earth at an altitude of ~80-100 km.

The spinels were extracted with a strong magnet from a suspension of the K/T boundary clay. The spinels were crystallized as octahedra in the range of 1-20 um in sizes. The magnetic minerals at both sites are principally nickel-rich (Ni>1%) magnesioferrite spinels with strong variation in the chromium content (0-10% Cr₂O₃). The octahedra from the K/T boundary sediment from Cerbara show a lower Cr content with lattice parameter of a=8.366Å than those of the larger particles (5-20 um) from Bjala with lattice parameter of a=8.345Å. However the smaller particles (1-3 um) from the latter site show low Cr content. The octahedral faces of the large particles of Bjala show etch pits. This etching could be a consequence of sulphuric acid rain following the K/T impact.