Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary

@article{Bohor1984MineralogicEF,
  title={Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary},
  author={Bruce Forbes Bohor and Eugene E. Foord and Peter J. Modreski and Don M. Triplehorn},
  journal={Science},
  year={1984},
  volume={224},
  pages={867 - 869}
}
A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are… 
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References

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An Iridium Abundance Anomaly at the Palynological Cretaceous-Tertiary Boundary in Northern New Mexico
An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater
Extraterrestrial Cause for the Cretaceous-Tertiary Extinction
TLDR
A hypothesis is suggested which accounts for the extinctions and the iridium observations, and the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other.
] makes a case for concentration of iridium in marine clays at the K-T ues for iridium of 5
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Gratz for their helpful comments; and C. Sanderson for laboratory assistance
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