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Techniques for CollectorsCleaning Ojuela Mine material

27th Nov 2020 18:49 UTCAndrew Haighton

I have a couple of pieces of Ojuela Mine material (Mexico) for which I would like to remove the Calcite.

The first piece has Adamite (including at least partial pinwheels), that although not huge (1.5 cm pinwheel at best), would be far better if exposed by removing the Calcite.

The second piece is a fairly large piece of vuggy limonite with Conichalcite and some Austinite on the top. Once again, although the Calcite is relatively nice, it detracts from the bright green of the other material, so I would like to remove that as well.

Looking on Mindat, I see that Adamite is easily attached by weak acids and that Conichalcite is soluble in HCl, which is easier for me to obtain than other acids such as Nitric or Oxalic.

I propose to use a higher grade of vinegar to clean up the pieces, starting with small portions of the aforementioned specimens to see if that will remove the Calcite and not affect the other minerals, and if that works go further.

Is there a better way, or has someone removed Ojuela Calcites before? Clearly HCl is not a thought (as noted above).

Thanks.

27th Nov 2020 18:55 UTCPaul Brandes 🌟 Manager

Any way to provide a photo or two to see what you're/we're dealing with??

My first thought might be to try Sulfamic Acid, but not sure what that would do to the specimens....

27th Nov 2020 20:02 UTCAndrew Haighton

Hi Paul;

I'll take some photos and post this weekend. Also, if there is Calcite around Dioptase (not on this particular material), is that also a problem to remove?

Thanks.

27th Nov 2020 20:32 UTCAlfredo Petrov Manager

Dilute nitric acid is the best for removing calcite from dioptase. Nitric in general is more gentle on silicates (like dioptase) than other acids are.

30th Nov 2020 00:45 UTCAndrew Haighton

Hi Paul,

I've uploaded a few photos of both pieces.

Where do you get sulfamic acid? I'm not familiar with that one.

Thanks.

29th Nov 2020 14:03 UTCAndrew Haighton

Here are a couple of photos of the conichalcite. The piece is about 100 mm in length, and if the calcite were removed, but not at the expense of the conichalcite, it would be quite a nice piece as it is quite vuggy and rich. Also some minor rosasite present, the teal parts.

29th Nov 2020 14:05 UTCAndrew Haighton

This is the back side of the same piece.

29th Nov 2020 14:11 UTCAndrew Haighton

This is the adamite piece with calcite, possibly some minor hemimorphite. The piece is 85 mm long. Not the best piece, but has some of the adamite hidden by the calcite, which I noted when under LW UV light. There is already a few nice small pockets and in the next photo what appears to be a covered pinwheel of adamite.

29th Nov 2020 14:17 UTCAndrew Haighton

I recognize that these are not the greatest photo, but you get an idea of the matrix and the general crystals with respect to one another. This pinwheel would be approximately 25 mm, so relatively large and worth exposing.

I took the photos on my tablet, which is not as good as my camera. What ever I try, I'll take better before photos, and comparison photos after the treatment, and post.

Thanks.

30th Nov 2020 03:26 UTCMark Heintzelman 🌟 Expert

Now that you have a good response for the mechanical means of your desired efforts, allow me to question it's necessity. These are classic association specimens from Mapimi, and I have to express the idea that removing these associated calcite won't markedly improve this material all that much, but it will erase evidence of this very notable association . . . is it really worth the effort?

As for the adamite, one my own cuprian versions with this associated calcite was improved sufficiently with just a little mechanical removal around the edges of the radial aggregates, while retaining all the rest (image prior to improvement) . . . can't imagine how dull and unimpressive it would be were I to entirely stripped off all the calcite. Food for thought.

11th Dec 2020 23:35 UTCStephen C. Blyskal Expert

Andrew,

I was not familiar with Sulfamic acid either, but Paul Brandes turned me on to it and I found it at my local Home Depot store here in Texas. If you are not in the USA you might have to search for it. Apparently it is used commercially and comes in 1 lb plastic buckets for less than $10. It is a powder, so you mix it with warm water. Removed calcite effectively from a Michigan copper specimen.

12th Dec 2020 16:20 UTCSteve Hardinger 🌟 Expert

Sulfamic acid is strongly acidic, so I would be concerned about its effect on the other minerals present. I would start with a milder acid first. There is a good study on selective removal of carbonates (by acid) in the November 2020 Mineral News that might be of some value.

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