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Techniques for CollectorsBlack coating on rhodonite

14th Oct 2014 02:13 UTCHoward Heitner

I have a rhodonite crystal in calcite from Franklin, NJ with a black coating. Presumably some manganese oxides. I can not use acids, because I want to preserve the calcite matrix. Any suggestions?

14th Oct 2014 07:28 UTCRock Currier Expert

If they are manganese oxides you might be able to remove them with a solution of acidified hydrogen peroxide.

14th Oct 2014 11:08 UTCHoward Heitner

Never heard of that combination. Would the dilute peroxide from the drugstore work? I don't think that the 30% stuff is sold to the public, because it could be used to make explosives. I think sulfuric acid would be the best choice to leave the calcite alone.

14th Oct 2014 17:45 UTCEarl Verbeek Expert

A solution that has gained popularity among the Franklin-Ogdensburg locals is 50% hydrogen peroxide (the weak stuff sold in drugstores) and 50% white vinegar. Cheap and effective on many specimens with brown to black coatings that develop upon weathering. A fairly short dip of a few minutes' duration should not much affect the calcite, but keep an eye on it. However, since the calcite in the specimen you photographed is still white, and hasn't developed that bronzy brown tarnish common to weathered, Mn-bearing Franklin calcite, the black coating on the rhodonite crystal may be natural. I've seen such coatings before, on rhodonite and bustamite crystals in unweathered specimens; presumably they are a product of low-T hydrothermal alteration. If that is the case, you may want to consider leaving the specimen as is. Also check to make sure the coatings is indeed black and not dark brown; if the latter it may be serpentine (common alteration product of rhodonite), and the peroxide-vinegar solution won't touch it.

Cheers- Earl

14th Oct 2014 17:51 UTCReiner Mielke Expert

"I think sulfuric acid would be the best choice to leave the calcite alone." Calcite dissolves in sulphuric acid!

14th Oct 2014 20:09 UTCHoward Heitner

I am not sure about the sulfuric acid. Wouldn't the calcite form a protective coating of gypsum. It all depends on the concentration. and what the final pH is. Perhaps sodium bisulfate would be better? If acetic acid works all the better

14th Oct 2014 23:40 UTCWayne Corwin

Howard

Sulfuric acid WILL attack calcite, just slower if deluted.

I have had good luck removing the black manganese oxides on Rhodonite with SIO (Super Iron Out).

It usually doesn't take more than an hour, and shouldn't hurt the calcite.

15th Oct 2014 00:12 UTCRock Currier Expert

Sulfuric acid will eat up calcite just like most other acids. Gypsum will not form, or if it does it won't till the acidity drops way way down.

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