Which chemical element was found unusually high in abundance in the clay layer at the K-Pg boundary?

The ''Strata'' (plural of Stratum) is a layer of sedimentary rock or soil or igneous rock that were formed at the Earth's surface. An unusual abundance of the chemical element 'iridium' (about 100 times than normal) in a layer of rock 'strata' at the Cretaceous-Paleogene (K-Pg) boundary is most significant at Raton, Mexico; Stevns Klint, Denmark, UNESCO World Heritage; the Chicxulub crater, Mexico; Yucatan Pininsula, Mexico; Trinidal Lake state park, in the Raton of Colorado, USA.

The K-Pg boundary marks the end of the Cretaceous Period and the beginning of the Paleogene Period. Its age is usually estimated at 66 million years. The K-Pg boundary is associated with the Cretaceous-Paleogene extinction event, a mass extinction which destroyed 70% of all species except birds.

A team consisting of the physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel were the first to link the extinction to an extraterrestrial impact event based on the observation that ''iridium'' is much more abundant in meteorites than it is on Earth.

Iridium have symbol-Ir and atomic number 77. It is a very hard, brittle, silvery-white transition metal. Ir is the second-densest metal (after Osmium) with a density of 22.56 g/cm3 as defined by experimental X-ray crstallography. Ir is hardening agent for platinum. It uses as alloy in pen's nib with osmium and in high temperature bearing crucibles.