FMF - Friends of Minerals Forum, discussion and message board

[Mathias]

Dear FMF specialists,

I've just acquired a small batch of mineral specimens, 10 pcs, half of them are from Peru, containing Tetrahedrite, Galena sphalerite, Rhodochrosite in various combinations.
But there was also a relatively large specimen supposedly Datolite among this batch. It however came without a label (alike the other 9 specimens but these are easily determinable regarding locality), meaning that I basically know nothing about it.

Besides colorless Datolite crystals, there is a small amount of another purple-colored mineral present. The backside of the specimen is completely filled with small Datolite crystals.

Is there anybody out there who has a clue where this specimen might be from?

Thanks and regards,
Mathias

[Kevin Schofield]

[Peter Megaw]

I agree with Kevin's calls on the associated species (I suspect the calcite is orange-pink fluorescent, which unfortunately doesn't narrow things down either). However, neither the habit nor the color of the datolite look like things I've seen from Charcas, and that DT scalenohedral calcite is uncommon from there as well.

[kushmeja]

Looks like Prospect Park Quarry in New Jersey, USA, or possibly one of the other NJ trap rock quarries. Datolite of similar appearance is found there, and with the same associated minerals too. I have at least 1 piece at home from Prospect Park that has nearly identical Datolite crystals as far as size, color & shape. I'm out of town the next few days, or I'd post a pic now. If you check Mindat though, there should be ample examples on there too.

[Mathias]

Dear Kevin thanks for your reply.

Could it perhaps be that the biger crystals, on the first photo, aren't datolite at all, but Calcite? I have some greenish USA (upper new street quarry)- and also Dalnegorsk Datolites in my collection but its difficult for me to compare these with the subject specimen and tell its the same crystallization, lustre, etc. just by the bare eye.

It could easily be tested with some hydrochloric acid, but I try to postpone the destructive analysis a little bit.

Thanks,
Mathias

[Peter Megaw]

I wondered that too...UV would tell too and it's nondestructive. Datolite is also harder (5+ vs 3) so a well placed pin will tell you a lot

[Kevin Schofield]

[Mathias]

Hi Kevin,
I didn't do any testing because the vendor said it is Datolite, but from where he didn't know, which I believed. Now, during our discusion, there"s doubt.
I'll do the tests,
Thanks,
Mathias.

[Peter Megaw]

The little green crystals in your second photo look more right for datolite

[Mathias]

Hi, I did an analysis, it is Calcite indeed. Nice pink color in UV light.
That's totally OK with me, I could have done this "research" before asking FMF, but I guess the specimen resembles Datolite sufficiently that I didn't initially doubt the mineral type. I also needed to buy a UV lamp, which I didn't have before.
The specimen could be from many locations, I'll leave that open.
Thanks for all your help because it led to the right determination.
Thanks again.
Mathias.