FMF - Friends of Minerals Forum, discussion and message board

[James Catmur]

I have had this for 45 years but have no ID or locality.

SG 2.43
Hardness 4 to 6
Streak black
Color black

[Pete Richards]

What can you say about the cross-section shape of the crystals?

[James Catmur]

Perfect rhombohedra (i.e. parallelogram) with no clear cleavage across the crystal

[Pete Richards]

Looks like a dark amphibole to me. The amphibole taxonomy has become impossible without access to fancy equipment, but earlier we would have said hornblende or maybe actinolite. If it is an amphibole, the cross-section shape will likely have corner angles of about 60° alternating with 120°, and there should be cleavages parallel to the visible faces, though the cleavages may not be obvious.

Someone who knows petrology better than I do can provide a name for the rock itself. Looks like it contains a lot of chlorite.

I doubt whether it can be pinned down to a specific locality, except perhaps by somebody who has personally collected the same stuff somewhere.

[James Catmur]

Pete

Yes looks like 60° alternating with 120°. The cleavage parallel to the visible faces is poor or not obvious.

I thought locality will be unknown, but who knows. Could be English

Thanks