FMF - Friends of Minerals Forum, discussion and message board

[Ning]

I thought I was really clever picking up this specimen for cheap in a Bangkok mineral shop. I thought it was calcite covering a ton of gemmy pink tourmaline terminated points. Put it in a vinegar bath and nothing happened. Not calcite then...

After a bit of investigation I think It may be Cookeite. I've had a few Acids recommended to me such as Hydrofluoric and Trifluoroacetic. But these look terrifying and not for a novice such as me, so that's a hard No. My only other option seems to be a light touch with a Dremel. Also slightly terrifying but I do have a steady hand.

Can anyone think of anything else?

[Jesse Fisher]

Unfortunately, cookeite is essentially a micro muscovite, and muscovite is largely impervious to most chemical treatments, even HF. The only way I have ever found to remove the stuff is by repeated soaking in hot water (which sometimes expands the mica) followed by a glass bead air abrasive. It's a slow process!

[Joseph DOliveira]

I agree, glass bead blasting may do the trick,
as the glass beads should not damage the tourmaline.

[alfredo]

Cookeite is a member of the chlorite group, not a mica. I've dissolved other chlorites (clinochlore-chamosite) in hot sulphuric acid, but whether that would also work on cookeite, I'm not sure.

[Jesse Fisher]

I am not sure that I would recommend using hot sulfuric acid any more than HF to anyone who does not have access to a "professional" grade laboratory setup and training in proper lab safety technique.
.

[Ning]

Thanks for the responses!

I live on a tiny island in Thailand, so I don’t have access to a glass ball blaster unfortunately. Though I do know a few chemists and biochemists on the island. Perhaps they could hold my hand of I go down the acids route. For now, I think my safest bet is to experiment at the edges with a Dremel with a flex arm and some small diamond burs.

I bought this as a fun project and I quite like the mystery of the rock if upon Dremel failure if I leave it mostly untouched.

[Paul Bordovsky]

The cookeite can be removed in some cases, by mostly mechanical means, but very carefully. This Kashmiri tourmaline was encrusted completely on the back side, and substantially on the display side. Leonard Himes prepared this for me using a high pressure water gun in conjunction with some other process. I don't think he used any dangerous or harsh chemicals. I think he did a very nice, skillful preparation. You might try contacting him and inquire about his services.

[alfredo]

Get it professionally analysed and make sure it really is cookeite, and not the closely related species borocookeite! When the pink tourmalines from Malkhan in Siberia started coming out, dealers quickly cleaned off the ugly white coatings on the tourmaline, which later turned out to be the new mineral species borocookeite, and every rare species collector in the world wanted a piece. But unfortunately they had almost all already been cleaned and the borocookeite thrown away. In North America only Excalibur Minerals in New York had saved some of that ugly crust from there, so they had a monopoly on sales of the new species. ;))

[Ning]

Thanks for the replies, that specimen is gorgeous! I’ve not seen a color combination like that before. Really striking!

Professional appraisal/cleaning Isn’t something I’d considered. I had a nice chat with the owners of a mineral lab in Singapore a while ago. I could ask them if they think it’s worth cleaning.

[Tobi]

I understand Alfredo's worries concerning the destruction of a new mineral species. But if that crust is not something new or rare, a professional procedure of removing it might reveal a really beautiful piece of tourmaline. The photos Paul posted above are a great sample that demonstrates how an encrusted chunk of rock becomes a top piece of a precious and very beautiful mineral!