FMF - Friends of Minerals Forum, discussion and message board

[Marcella33]

Hey everyone. I found this stunning mineral while hiking around in northwestern South Dakota. I thought the mineral could possibly be gypsum. What do you think? I know it's difficult to identify a mineral without having the piece in your own hand.... but any thoughts on this would be great!! Thanks!


(P.S. I recently took these photos in the Shenandoah Mtns. but the minerals were found in desert prairie bluffs)

[Bob Carnein]

It's almost certainly twinned gypsum. Try the hardness--can you scratch it with your fingernail?

[Pete Richards]

I agree with Bob - it's gypsum..

If you have one to sacrifice, I'd try to split it down the middle parallel to the biggest flat surface - the surface we see in the first image. It should split almost as nicely as mica - quite easily - to produce a nice flat clean surface on which your fingernail test will be more obvious. This will also demonstrate gypsum's perfect cleavage, an extra confirmation of its ideintity.

This kind of growth is fairly common in gypsum that grows quickly, though what I am familiar with is not quite so shaped like a Christmas tree. The branches on one side are twins with the branches on the other side.

[James Catmur]

Not that uncommon. The Romans used to mine large sheets of it to make 'windows' and I visited such a locality where sheets 60cm across could be collected (and split into thiner sheets)

[Sante Celiberti]

These formations are quite common in Tuscany.
The rough surface that is observed in your crystals is due to dissolution, as Gypsum is an easily soluble mineral.
Only samples that have not been in contact with water or other fluids retain a flat, smooth surface.
You'll find this last surface when you take the test Pete suggested.
Most likely you will not be able to obtain a clean split of the entire specimen since the contact between the single crystals, both in parallel and oppositely twinned formation, represent an easy breaking point.
Nevertheless, both the easy cleavage referred to by Pete and the translucency/transparency of which James speaks and which led the ancient Romans to define this mineral "Lapis specularis" will be evident.

Good luck.
Sante

[Marcella33]

Thank you everyone for your replies!! I had no idea this form of gypsum was so common. I only brought one piece of it home with me so I don't want to split it apart, but I'm glad that I can still pretty confidently say that this is Gypsum based on everyone's agreeable observations.

- Marcella