FMF - Friends of Minerals Forum, discussion and message board

[nightengale]

Hello! This is my first post on your site. I'm seeking some information about septarians. I grew up finding these wonderful rocks on my dad's farm, and I'm trying to find out how they were formed, what they are made of, etc. The problem I'm having is that I keep hearing about septarians from Madagascar, Utah, and the UK. I haven't read anything about the septarians from Kansas; which are a little bit like the pictures that are posted, yet are somewhat different in color, pattern, and size. Ours range from rocks you can put in the palm of your hand to boulder-sized. Do you have any information about "our" septarians? Thank you so much!

Meredith

[John S. White]

As is often the case there is a reasonably good description of septarian concretions or nodules at Wikipedia, suggest all who are interested give it a look. One of the most interesting examples in my diverse collection consists of cabochons about 2 cm long of pyrite filling cracks in black apatite. They are very attractive. If I can I will take a photo tomorrow.

Jordi would hate it! This gives me great pleasure, almost as much as the USA beating Spain in the World Cup.

[Pete Richards]

As I understand it, septarian nodules form in soft sediment, often mud, in the same way as other concretions do - conditions at various spots in the mud become favorable for the precipitation of minerals such as calcite, siderite, and dolomite, and these minerals are deposited in the spaces between the particles of mud. The process works its way outward, forming a concretion. They are typically round and often flattened parallel to the horizontal layering in the mud, because fluids percolate more easily in that direction. In the case of septarians, a good bit of water is trapped inside the concretion, and as the water gradually seeps out, the now-stiff concretion shrinks and cracks. The cracks are largest in the center, typically.

Eventually, the mud turns to sedimentary rock, and the cracks in the septarian nodule typically become filled or partly filled with minerals. The minerals are often more resistant to erosion/dissolution, in which case they wind up sticking up from the overall surface.

Septarian concretions found in Ohio in late Devonian rocks are often 6 feet across! The mass of the concretion is probably mostly siderite - they dissolve very slowly. The septaria are mineralizes with up to four generations of ferroan dolomite, along with calcite, quartz, Baryte, apatite, and whewellite. Microcrystals of marcasite that look like tufts of grass and of pyrite with right-angle bends are sometimes preserved as inclusions in the Baryte and quartz. Pretty neat stuff!

Septarian concretions from the Pennsylvanian coal measures of western Pennsylvania contain cracks that are open or filled with calcite, but of great interest are the rosettes of wurtzite that can be found in them, sometimes in epitactic relationship with sphalerite.

[nightengale]

Thank you ! I will give Wikipedia a look!

Meredith

[nightengale]

Wow...thak you for that incredibly detailed reply! It sounds like our septarians had a huge process to go through before they became these incredible rocks. Our rocks are usually rounded, and we get really excited when we find one that is "prefectly" round. (We found one last week!) I can't say that we have anything approaching 6 ft., though.

Thank you!

Meredith

[John S. White]

While technically not a septarian, this is an interesting example of cracking with subsequent infilling of another mineral. This cabochon is from Indonesia; the black is apatite and the mineral in the cracks is pyrite. The piece is 22 mm long. Sorry the photo is not better, it is hard to photograph.

[alfredo]

John, Any idea what kind of environment your indonesian apatite/pyrite formed in? It certainly is unusual.

[Pete Modreski]

Meredith,

Do your septarians from Kansas actually contain a pattern of hollow places and mineral-filled cracks in the interior, like the classic septarian nodules from Utah or elsewhere? (as opposed to just being round, solid nodules) I think that most of us have never seen such nodules from Kansas; perhaps you can post a picture of one that has been sawn or broken in half.

Sincerely, Pete

[John Jaszczak]

Pete Richards kindly took me to Donahoe Station, Pennsylvania many years ago. It was great fun and yielded many great micromounts. Here is one of my favorite trigonal wurtzites- actually one I found in processing septaria Pete gave me.

[hettybrah]

Found this in a google search. I live in Eastern PA and recently moved near a bunch of strip mines. Two separate friends from separate circles told me about "amethyst geodes" they found in the area that were 3-4ft long.

I did a bunch of exploring and came across these nodules, which were VERY HARD...this one was loaded with pyrite, but the "pocket" seems to be not even scratching the surface of the actual concretion. The nodule just laughed at my 20lb "persuader" sledge. This thing was harder than even the herkimer diamond rock up in NY!!!! Any suggestions on opening them up? How the heck you you cut a slab out of a 4ft rock?!?!?

[hettybrah]

I did find a smaller one(2ft) that was broken open, and managed to chisel out a small thin slab of "the core"....it had 3 one-inch pyrite cubes in it...badly damaged due to my extraction methods, though :/

[hettybrah]

partial pyrite cube that came from the smaller broken nodule.