Septarian Nodule or Concretion

 


 

round septarian nodule filled with yellow calcite crystals. On a black background

 

Septarian from the Latin Septum


A septarian nodule is a type of rock whose name comes from the Latin word septum meaning a dividing partition between two tissues or cavities.  Today the word septum is used to describe the fleshy partition in our nose that separates one nostril from the other.  This distinctive stone which is easy to identify takes on a high polish and should correctly be described as a rock not a mineral because it's made up primarily of three different minerals.  The nodules are sometimes likened to prehistoric mud balls because of the way they formed.

A septarian nodule is composed of the mineral calcite (yellow centre), aragonite or siderite (the brown lines) and limestone which is the outer rock.  

 

 

Nodules or Concretions?


Although often known simply as septarian, it's more accurate to refer to these structures as nodules or concretions.  Both are quite similar hence the terms tend to be used interchangeably.  In geology a nodule refers to a small irregularly shaped mass or lump of crystals or particles with a contrasting composition which means they're not all the same and may even be separated from the formation in which they occurred. A concretion is a self contained body that's enclosed within sediments that are generally softer and of the same composition as the formation in which they originated.  Concretion also comes from Latin and means 'to grow together' or 'to harden' and these structures are often looked upon as geological curiosities because of the many unusual shapes, sizes and compositions in which they occur.

Septarian nodules are believed to have formed in shallow lakes as the tide caused an accumulated mass to gently roll back and forth and in doing so, it built up layers of sticky mud which subsequently dried out during the hotter months as the water receded. The mud balls were then buried under sediment and any cracks that appeared slowly filled by means of seepage with a coarse crystalline substance such as silica or calcite (from the shells of dead marine creatures). The crystals which formed as a result are the bright yellow centres of the septarian nodule.  A thin wall of calcite was also transformed into aragonite or siderite which separated the heavy clay exterior from the crystallized centre.

 

 

septarian nodule in a museum display cabinet

 

 

 

Formation of a Septarian Nodule


Septarian is known to have formed during the Cretaceous Period which began approximately one hundred and forty five million years ago and ended sixty six million years ago around the time the dinosaurs disappeared.  They formed in bodies of water as minerals and organic matter accumulated around a centre mass and gradually over millions of years and with the help of various geological processes, hardened and became cemented together forming a nodule.  The minerals are likely to have included sandstone which is compacted grains of sand, shale which is compacted mud, siltstone (fine grained silt, sand, clay or other materials carried by water and subsequently deposited as sediment) and limestone which is primarily calcium carbonate.  The mineral aragonite is the crystal form of calcium carbonate which is a chemical compound that occurs naturally in rocks most notably as limestone.

Whilst the exterior of the septarian nodule was hard and featured a network of ridges, the interior contained distinctive angular cavities or cracks known as 'septaria' from the Latin word septum.  It's believed these were caused by the dehydration and shrinkage of clay although some suggest they may have been caused by the expansion of gases generated by decaying organic matter in the centre of the nodule.  They may also have been caused by fracturing or shrinkage caused by earthquakes or compaction.

Irrespective of how the cracks formed, they subsequently filled with hardened natural minerals as groundwater changed and receded.  The minerals in the vast majority of cases were silica or calcite. 

Describing precisely how septarian nodules formed is not easy and there are many different views and interpretations.  Numerous questions remain unanswered and it's an ongoing topic of debate amongst geologists however the general formation process is one that's relatively common in sedimentary rocks

 

 

two polished septarian nodules, the one in the foreground is slightly larger

 

 

Moeraki Boulders


The moeraki boulders are large grey coloured septarian concretions that can be found on a stretch of coastline in New Zealand.  The boulders which were buried for millions of years beneath mudstone began to appear because of erosion from coastal cliffs.  In years to come more will emerge from the mudstone as geological changes continue to reshape the landscape.  It's believed the largest concretions took about four million years to get to their current size.

Spherical boulders that are almost identical can also be found close to Hokianga Harbour on the North Island and similar concretions can be found in many countries around the world.  The Moeraki boulders are famous primarily because of their spherical shape and size.

 

 

Large spherical stone structures on the beach. Known as the Moeraki Boulders


Click for Additional Photo

 

 

Septarian in Crystal Healing


In crystal healing and according to Judy Hall's Crystal Bible, a septarian nodule is said to encourage one to take care of the earth.  It harmonizes emotions and is generally a joyful stone which helps to bring new ideas and the enthusiasm to follow them through. It enhances the ability to communicate within a group and is emotionally nourishing and calming.

 

 

Our Photographs


The photo of the septarian nodule at the top of the page is courtesy of Stan Celestian (Flickr), the piece in the second photo is on display in London's Natural History Museum, photo by Stone Mania.  Click the images to see the original photos.  The two polished spheres are from our own collection.

 

 

Our Collection of Septarian

 

clickable shopping trolley

 

 

Further Reading

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