Wednesday, February 29, 2012

March 2012 Rock of the Month

 Septarian Nodules

 Septarian concretions or septarian nodules are concretions containing angular cavities or cracks, which are called "septaria". This word comes from the Latin word septum; meaning “partition", and refers to the cracks or separations in this kind of rock. Septarians formed somewhere between 50 to 70 million years ago as a result of volcanic eruptions. Dead sea life was then chemically attracted to the sediment around them, forming mud balls. As the oceans receded, the balls dried and cracked. Due to their bentonite content they also shrank in size, creating the cracks inside. As decomposed shells seeped down into the cracks in the mud balls, calcite crystals formed. The outer thin walls of calcite then transformed into aragonite. Septarians are composed of Calcite (the yellow centers), Aragonite (the brown lines), and the outer grey rock is Limestone. Septarians can be found all over the world in many different locations.