FMF - Friends of Minerals Forum, discussion and message board

[Jolyon Ralph]

An interesting discussion has started on mindat about the black 'helvite' crystals from China - it seems to be they may not be helvite at all. Romanechite or other manganese oxides possibly pseudomorphing helvite is the leading theory at the moment.

Does anyone here have any more information about this subject?

Please see: https://www.mindat.org/mesg-19-266170.html

Thanks.

Jolyon

[lluis]

Hi, Jolyon

Few I can add, except that, as has been said many times, manganese oxides are a black beast, and depending how analysis has been carried, could be romanechite or any other manganese oxide.

I had a talk with Jordi about the blackening of Chinese helvites a loooong time ago.

Personally, I find no reason to believe that the black manganese oxide is romanechite (where the Ba comes from?). Wad would be a better guess, done its facility to dissolve....

With best wishes

Lluís

[Jordi Fabre]

Interesting discussion. What I can say is that for what I talked in the past with Museum curators and collectors highly specialized in rare minerals, it seems that the "helvites" from Jinlong are currently manganese oxides and it is totally impossible to know what it was at the start. Helvite? Genthelvite? Danalite? Who knows...

By my personal experience these current "traces of something" are extremely unstable, but not only to any kind of acid but also to running water, ultrasonic cubes, water gun.... so they are probably just "relicts" of something that just maintains the former structure.

[Peter Megaw]

I can understand the physical instability, but these apparently dissolve without residue which is harder to understand

[alfredo]

John Attard advised me that the best thing to clean manganese oxide crusts off of minerals is slightly acidified hydrogen peroxide. I just tried it on a dirty weathered mixture of rhodonite + jacobsite, originally banded pink and brown, but weathered completely black. One glass of weak (3%) H2O2 from the medicine cabinet, added a couple teaspoons of dilute HCl, dumped the black rock in. Intense effervescence (oxygen most likely), and within 5 minutes the black crust was gone! No mess, no residue, the solution remained clear and colourless... Given the right chemical conditions (weak acidity and an oxidizing environment) manganese oxides seem to be amazingly soluble.

[Peter Megaw]

Very handy recipe Alfredo, thanks to you and John. Blackened specimens of Santa Eulalia rhodochrosite crusts and Mn-calcite from Colorado might profit from it.

Is there any post-tretment neutralization...beyond a good soaking...needed?

[alfredo]

I soaked it for a couple hours, changing the water a couple times. Seemed to be enough. No stains after it dried.

However, even though the acid is quite weak, I'd still hesitate to use it on calcite or aragonite. Probably OK for rhodochrosite.

Rudy Tschernich, in his encyclopedic book on Zeolites, recommends ascorbic acid ("vitamin C") for removing manganese oxides. Some health food stores sell ascorbic acid in pure powder form in 100 gram bottles, a lot cheaper than buying it in pills. I have not tried this yet.