Jurassic Septarian Nodule - National Museum of Slovenia - Ljubljana - Rock and Mineral Displays on Waymarking.com

This Jurassic Septarian Nodule is located outside the service entrance of the National Museum of Slovenia on Prešernova cesta, Ljubljana.

"Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, called "septaria". The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock. There is an incorrect explanation that it comes from the Latin word for "seven", septem, referring to the number of cracks that commonly occur. Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of cement precipitated.

The process that created the septaria, which characterize septarian concretions, remains a mystery. A number of mechanisms, e.g. the dehydration of clay-rich, gel-rich, or organic-rich cores; shrinkage of the concretion's center; expansion of gases produced by the decay of organic matter; brittle fracturing or shrinkage of the concretion interior by either earthquakes or compaction; and others, have been proposed for the formation of septaria (Pratt 2001). At this time, it is uncertain, which, if any, of these and other proposed mechanisms is responsible for the formation of septaria in septarian concretions (McBride et al. 2003). Septaria usually contain crystals precipitated from circulating solutions, usually of calcite. Siderite or pyrite coatings are also occasionally observed on the wall of the cavities present in the septaria, giving rise respectively to a panoply of bright reddish and golden colors. Some septaria may also contain small calcite stalactites and well-shaped millimetric pyrite single crystals.
Moeraki Boulders, New Zealand

A spectacular example of septarian concretions, which are as much as 3 meters (9.8 feet) in diameter, are the Moeraki Boulders. These concretions are found eroding out of Paleocene mudstone of the Moeraki Formation exposed along the coast near Moeraki, South Island, New Zealand. They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stage quartz and ferrous dolomite (Boles et al. 1985, Thyne and Boles 1989). Very similar concretions, which are as much as 3 meters (9.8 feet) in diameter and called "Koutu Boulders", litter the beach between Koutu and Kauwhare points along the south shore of the Hokianga Harbour of Hokianga, North Island, New Zealand. The much smaller septarian concretions found in the Kimmeridge Clay exposed in cliffs along the Wessex Coast of England are more typical examples of septarian concretions (Scotchman 1991)."

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Type of Display: Geological

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