Anyone know what this is?

[pwhole]

I recently photographed this strange formation in a Castleton cave passage and don't have much idea what's going on here - is this fossil-based, water-worn or what? It's not on a normal caver route, and so won't be seen very often, but I just wondered if this was a one-off, or there are other formations like it?

It seems to have a 'skin' of mineral over some other mineral, or possibly limestone - hard to tell. I'm wondering if it's some strange baryte formation. The area photographed is about two feet across, and the formation 'merges' into normal limestone and vein mineral at the base. The passage was visited by lead miners, so it's possible the 'skin' was broken off during their work.

 

[Brains]

Hard to say from just a couple of pics, but with the presence of a mineral vein this offers a few options.
- The vein MAY be on a small fault, which has developed mineralised slicensides, with further overgrowth in a void
- The mineral would have grown into a void from a flowing mineralising fluid. The mineral could therefore by responding to turbulence in the fluid in its growth pattern.
- The top picture shows a series of parallel lines in the mineral - what is the current orientation of this, if horizontal it seems to resemble the texture on flowstone from a mineralising fluid flowing down a surface in a vadose environment.
-A combination of these and other factors I havent thought of...

 

[gus horsley]

I'm not sure what the underlying formation is, possibly an aligned conglomeratic lens in the limestone.  Some scale would help.  This appears to have a calcite overgrowth and a crust of what looks like a manganese mineral (the dark stuff), probably psilomelane or pyrolusite, both of which behave similarly to calcite flowstone in that they leach from surrounding rocks and are redeposited on wallrock, sometimes forming black, slightly opalescent stalactites.

 

[Pitlamp]

It almost looks like some Blue John I've seen. Is it under Treak Cliff hill?

 

[pwhole]

It's a little south of Treak Cliff - it's in Pit Top Passage in Speedwell, above the Bottomless Pit, so it's on Faucet Rake, though the vein is mostly calcite in the accessible passage. As for scale, as I mentioned, it's about two feet across, and I'm looking downwards onto it at about 45?, so it's kind of a 'bulge' out of the main wall. The 'horizontal 'grooves are therefore probably also at 45? to the main wall and floor. I showed it to Jim Rieuwerts yesterday (who's no slouch at geology), and he said: 'What is that?' So if he was unfamiliar with the general shape, then I thought it might be worth chucking it out to a wider audience! What intrigues me is that in the cropped photo, there are 3 'layers' visible, almost like an onion-skin.

 

[gus horsley]

It's the cropped photo which made me think of manganese minerals as I've seen very similar deposits elsewhere.

 

[AR]

My first thought on seeing it was that it's a flowstone deposit, though the blue colour is unusual. As Gus suggests, maybe manganese in small amounts would give a blue rather than the more usual black, there's an outside chance it might be iron phosphate causing the colouration but I'd be surprised if it was that. Another possibility is a very thin layer of blue fluorite and/or galena overlaid by calcite?