10.3" Polished Septarian Slab - Utah

This is a large, 10.3" wide 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" which have become 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. Another is the cracks being due to the expansion of gases produced by the decay of 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) and aragonite (brown) and sometimes pyrite causing the very interesting patterns, which have often been described as 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
10.3 x 8.1", .6" thick