± 66 million yrs. BC

A mass extinction has taken place at the K-Pg boundary. The impact of a meteorite, ± 66 million years BC, created a crater of 180 km near the town of Chicxulub, Yukutan, Mexico. The impact has had major consequences for the climate and life worldwide.

Among other things, the dinosaurs and the ammonites were most likely extinct due to this impact. Many species of corals, sea urchins and bivalves did not survive this period.

An iridium anomaly has now developed in many parts of the world on the K-Pg border. This is an unusual deviation in the amount of the element iridium. Iridium is very rare in the Earth's crust, but in meteorites and comets, it occurs in large quantities. The discovery of a layer with abnormally large amounts of iridium is therefore seen as evidence of a meteorite impact on Earth.

In addition to iridium, small spheres, so-called microcrystals, and shocked quartz grains were also spread all over the world. Both microcrystals and quartz grains have been formed during the impact.

This clay with microcrystites was created after the impact of the meteorite in Mexico. The clay contains impressions of so-called microcrystites that were created by the impact. These are visible as dimples in the surface.

± 66 million yrs. BC to ± 2,58 million yrs. BC

A geode is a geological secondary formation within sedimentary and volcanic rocks. The name comes from Ancient Greek ‘geodes’ (= earthlike). They are hollow, vaguely spherical rocks, in which masses of mineral atter are secluded. They may include crystals. The crystals are formed by the filling of vesicles (= cavities of gas in the rock structure) in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids or by the dissolution of syn-genetic (= minerals formed together who frequently have inclusions of each other) concretions and partial filling by minerals precipitated from water, groundwater, or hydrothermal fluids.

Geodes can form in any cavity. They can form in gas bubbles in igneous rocks such as vesicles in basaltic lava. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface, if they are composed of resistant material such as quartz.

Visible bands corresponding to varied stages of precipitation may show patterns that reveal points of fluid entry into the cavity, when cut in half. Varied colors corresponding to changes in chemistry could be visible too.

Geodes are found where the geology is suitable. Many of the commercially available ones come from Brazil, Uruguay, Mexico, and Namibia.