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[Joan Massagué]

I have an old Skutterudite from Morocco with large, lustrous crystals, some of which with a thin crust of calcite partially covering the faces. How could I remove this calcite without risk of damaging the skutterudite. Would dilute hydrochloric acid be fine?

[Jordi Fabre]

Hi Joan,

Skutterudites are frequently etched by hydrochloric acid and although the first appearance after this process is great, looking very shiny and clean, after a short period, the Skutterudite's luster decay and they becomes dull.
Plus the Calcite appears frequently as an intrusions on the Skutterudite so, if you delete it completely, many times unaesthetic cleavage lines appears where the Calcite was, giving the sample a not so nice appearance.

The best to do would be to remove the Calcite just with a mechanical action, but you can also try to delete it not with hydrochloric but with a very soft acid, stopping the acid's action when the Calcite deleted arrived to the level of the Skutterudite faces, avoiding then to delete the Calcite inside the Skutterudite and with the extra advantage that the remaining Calcite will act as a neutralizer of the remaining traces of acid.

Jordi

[lluis]

Good afternoon

Agreed with what Jordi says.

But being a little curious, I wonder if the calcite could not be preferably etched without damaging the skutterudite by complexation.

If you can find some product like Viakal (sure there are any similar in USA), that will take out the calcite and let the skutterudite undamaged. And if it beguins to make any damage, you will see inmediately due to the dark blue colour that the soution will take (cobalt complex).

I am pretty sure that the skutterudite will not be etched, because the solubility product of such species is very, very low.

In order to avoid the damaged by removing the calcite inside the skutterudite, you could try what I did (not my idea. Read in any book devoted to mineral cleaning...):

Cover the parts you want not to be etched with butter.
It is easy to remove with warm water and detergent...

That is why I etch the parts I want in the pieces of benitoite, joaquinite and neptunite included in natrolite ....(just using hidrochloric acid as etching agent, of course)

With best wishes

Lluís

[Joan Massagué]

Jordi, LLuís,

Thank you for your advice, in light of which I'm not going to mess with this piece but just leave it the way it is!

[lluis]

Dear Joan

I think that I would do same...
By the way, a nice example.

Do you have any piece form Djebel Iuotom?
They are included in what I think is a quarzite (arenisca....), but are euhedral isolated crystals.
Nice ones for my taste.....


Best wishes

Lluís