6.9" Polished Septarian Slab With Crystals - Utah

This is a beautiful polished slice from a septarian nodule. This septarian was mined West of Orderville, Utah and was deposited during the Late Cretaceous, about 100 million years ago. The contrast between the grey limestone, yellow calcite crystals and brown aragonite is quite stunning. It's been polished on one side and displays well on the included display stand.

Septarian or septarian nodules are concretions containing angular cavities or cracks, called "septaria", filled with calcite and aragonite. A concretion is a hard, compact mass of rock that often forms around decaying organic matter. In the case of septarian nodules, the concretions formed around decaying sea life in a marine environment.

The exact mechanism for how the cracks form in the concretions is a mystery. One possible mechanism is the dehydration of the clay-rich core of a concretion, causing it to shrink and crack. The cracks could also be caused by gas expansion produced by the decaying organic matter within a concretion. Earthquakes have also been suggested as yet another mechanism.

The cracks in the concretions are then filled in with minerals such as calcite (yellow), aragonite (brown), and sometimes pyrite, causing very interesting patterns. They have often been described as looking like dragon's skin. They are frequently found as geodes with hollow, calcite crystal-filled cavities. More rarely, the fossils that originally started the formation of the concretion are still preserved in the septarian.
West of Orderville, Utah
Frontier Formation
6.9 x 6.7", about .6" thick