FMF - Friends of Minerals Forum, discussion and message board

[nurbo]

Hello,
My favourite thing is to travel the 10 miles or so from my home to get covered in mud, nettle stings and scratches from various bushes to collect Septarian stone from a brook that runs out of the local fells and through the village of Roeburndale in Lancashire UK. These really are spectacular looking things. I cut and polish them and have a huge collection.
They date from the Carboniferous period. There are many explanations as to how they form but as far as I know there is no definitive one. Basically ancient seabed dries, cracks and the cracks fill with heavily mineralised water which creates crystals inside them. There are so many different explanations of what these crystals are. Clearly calcite is present, also what looks like aragonite, Ive seen pyrite in them and also what seems to be quartz.
Many of them contain Ammonites and I have one which I will post seperately in the "what is it?" thread as it has something very unusual inside it. The ones here are a fairly good selection of what we find regularly up there. Locally they are sometimes known as Turtle Stones.
Just thought Id share this with you all.
b

[Pete Modreski]

Nice pictures of your septarian, "b"; it is good to see them to compare with our septarian nodules from Utah. I suspect most of us here (this side of the "puddle") are not much familiar with those from the UK. They look quite similar, though ours, Cretaceous age, rarely contain fossil shells. One friend of mine has a remarkable one that has a piece of dinosaur bone included in its center!

The Utah septarians usually have two generations of calcite in them; a brown first-formed layer lining the separated rock fragments, and a later, yellow layer deposited on them, which grades into yellow rhombohedral crystals if the central area remained hollow. The two layers differ in their fluorescence. I have done XRD on both layers of calcite, and in the Utah septarians both are indeed calcite; neither is aragonite as some people have supposed.

Best regards,
Pete Modreski, Denver, CO

[Pete Modreski]

P.S., here is a picture of the one polished septarian nodule from near Orderville, Kane County, Utah that I have seen, which contains a fragment of dinosaur bone. It also clearly shows the two successive, brown and yellow, generations of calcite that I described before. The bone fragment is surrounded by both layers of calcite, just like the rest of the rock fragments making up the concretion.

To the best of my knowledge, the parent rock the makes up the nodule is a marlstone (calcareous mudstone).

Pete Modreski

[nurbo]

Thanks for that Pete, the 'dinosaur bone' septarian is very impressive. I had read somewhere that the brown layer was also calcite, but assumed it was someone geting baffled by aragonite having the same equation as calcite as its a polymorph. Good to know the results of the xRd

This one came from the same location and we've been trying to figure out what it is without chopping it up, it looks like a large shell creature but that may be coincidental.