FMF - Friends of Minerals Forum, discussion and message board

[Steve Cole]

Found on the 'South Downs' in Southern England, this is very different to the usual chalk and flint in that area. It is 5cm across, rather heavy, not influenced by a magnet, and covered in triangular crystals. It cannot be easily scratched by a pocket knife.

[kushmeja]

Based on it's appearance and the minerals in that area, I'd say that it's pyrite that was partially altered to limonite. Mindat has pics of a pyrite nodule in flint that was found in West Sussex that looks very similar, but not altered to limonite.

[Peter Megaw]

Looks like a partially oxidized marcasite nodule from a sedimentary formation. It probably smell sulfurous if you take a whiff.

It will almost certainly continue oxidizing and eventually fall apart. You might be able to slow that down by washing with 90% isopropyl alcohol to kill the bacteria that hasten the oxidation. Scrubbing with a toothbrush to remove the rusty oxidation may reveal some of the golden metallic color of the marcasite.

[Joseph DOliveira]

I'm with Peter on marcasite, very similar to the marcasite nodules from the Belle Fourche Reservoir in South Dakota. If you're looking to clean them up and remove the oxidation, a light blasting with a sand blaster and glass beads will do nicely.

[Steve Cole]

[Roger Warin]

Hi,
Yes, a marcasite nodule. Radiating crystals.
These concretions are frequent in the north of France in farming areas. Dimensions up to 15 cm in diameter