FMF - Friends of Minerals Forum, discussion and message board

[Forrestblyth]

Unknown banded mineral from shores of lake Buttermere, Cumbria. UK

[Bob Carnein]

In order to ID this, you should tell us the hardness (can you scratch it with a fingernail; does it scratch a piece of glass or a knife blade; etc.) and powder a sample and put the powder in some hot vinegar to see whether it bubbles. To me, it looks like weathered travertine, which is calcium carbonate. Hardness would be 3 (slightly harder than a fingernail) and it would bubble in dilute acid (vinegar).

[Kevin Schofield]

Looks like either calcite or quartz that has grown in a fault gouge, crystallizing in a pocket between the handingwall and footwall, changing orientation as the fault moved.

The tests that Bob suggests will distinguish quartz from calcite

[Pete Richards]

My guess is that it is quartz, based on the appearance of the columns in close-up. It could also be barite or gypsum, which are known to form similar stretch textures.

[Mathias]

It's Pyknite.

[Roy Starkey]

Hi Mathias. Sorry but no, That is most unlikely.

The most plausible explanation is that the specimen shows typical crack-seal / crack-heal quartz which is of common occurrence in the Lake District.

A simple test with dilute acid to eliminate calcite / carbonate and a hardness test (will the white phase scratch glass) is all that is needed to confirm quartz.

Best wishes

Roy