Thermal decomposition pattern and particle size estimation of iron minerals associated with the Cretaceous-Tertiary boundary at Gubbio

Abstract

Mossbauer studies of the samples from the Cretaceous-Tertiary (K-T) boundary layer at Gubbio, Italy show that iron appears mainly in two phases, magnetically ordered hematite and a paramagnetic silicate phase. The average particle size of hematite is estimated to be in the range of 16 to 27 nm from transmission electron micrographs and lack of a Morin transition. The hyperfine magnetic field at the iron nucleus is observed to be somewhat less than that of bulk hematite, which may be explained by collective magnetic excitation. Stepwise heating up to 1000 ° C shows a decomposition pattern of the paramagnetic phase, which suggests it to be a tri-octahedral layer silicate. The iron-bearing phases found in the bulk sedimentary K-T boundary material are different from those found in the spherules separated from this material indicating that the redox conditions changed rapidly after the impact, becoming more oxidizing during the period these bulk phases were formed.