FMF - Friends of Minerals Forum, discussion and message board

[al mar]

Hello,

I have seen that rhodocrosites from some deposits, as the ones from Huelva province in Spain, or the El-Valle Boinás ones ( https://www.mindat.org/gallery.php?loc=106025&min=3406) had a very pale pink tone, or lack from any coulor. I also have seen an analyzed Mn carbonate with high Mn (much more than Ca and Fe) content and grayish colour coming from Peru.

On the other hand, some manganoan calcites show a quite intense pink colour, and rhodocrosites from other deposits have a vivid red colour.

Anyone knows why this happens?

[SteveB]

Isn’t it a particular Mn compound rather than just Mn itself? Or maybe i dont understand what you’re asking?

[Roger Warin]

Hi,
Atoms are not colored balls that stain a mineral.
Color is a physicochemical property difficult to understand by the amateur.
The hue results from the influence of electronic clouds that envelop the cations. It is therefore a physical phenomenon that depends on the field of ligands.
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of complex organometallic coordination.
Thus, the Cu2 + cation bound to water (H2O) is blue.
In the absence of water, it is colorless and in the presence of ammonia NH3, it is dark azure.
This effect is related to a ligand field.
We have three possibilities: Vacancy without ligand, or + H2O or + NH3.
For manganese it is the same. It is even more complicated, because the degree of oxidation intervenes, that is to say the valence.
The valence can vary from 2 to 3 and more. State 3 is stabilized by the complexes, i.e., when other ligands are added.
The hue of manganese cations is therefore variable depending on the environment.
An artificial derivative is Mn violet, which is a manganese and ammonium pyrophosphate.
Some compounds are black.
If for the amateur the question is obvious, the answer is less.

[Pete Modreski]

Al, Because rhodochrosite can contain other metallic elements besides Mn, particularly Ca and Fe, it's the purity of the rhodochrosite that mostly determines the color. Ideal rhodochrosite is pure MnCO3. One of the best articles that gives actual data on this is by Karen Wenrich in Mineralogical Record's 1998 issue about the Sweet Home mine. Her article was "Sweet Home [Colorado] Rhodochrosite-- What Makes it so Cherry Red?" (1998, vol. 29, no. 4, p. 123-127). She found that, for the Sweet Home mine, the deepest red and most transparent rhodochrosite had the purest MnCO3 composition (highest manganese content). Rhodochrosite that is cloudier and a less vivid pink, and then grading into brownish and grayish shades that are only barely pink, such as found at some other mines in Colorado and elsewhere, has lower Mn and higher calcium and iron contents, and it should be the iron content especially that makes the color more tan or brown.

[al mar]

Hi, to all, and thanks for the answers.

I found interesting the article that Pete quotes, and seems logical looking for example the capillitite variety,( https://www.mindat.org/min-27208.html ) impure and not pink. But seems strange that some analyzed rhodocrosites with more Mn than Ca appear more pink than manganoan calcites, with less Mn. If Mn causes the colour, and It´s present in the same oxidation state .

[alfredo]

Rhodochrosites in Bolivia tend to be brown and difficult to distinguish from siderite. I suppose this is due not just to the fact that Fe replaces some of the Mn, but also to the oxidation state of the Fe. For example most collectors expect ankerite to look brownish, but fresh ankerite crystals from the sodalite mine in Bolivia are pale green without any hint of brown color.

This discussion reminds me of a statement by my Mineralogy 101 professor a long time ago, "Color is the LEAST reliable property for mineral identification." (...but nevertheless the first one that everyone notices.)

[al mar]

Regarding to Bolivian ones, in Mindat there is one photo from San José, and I have seen botryoidal ones from Llallagua and one from Huanuni. All of them were pale pink, different to siderites from there.

[alfredo]

In 12 years working in Bolivian mines, I never once saw a pink rhodochrosite, only brown ones, but that of course is meaningless - There may well be many things I never saw.

The alleged pink rhodochrosite from the San Jose mine is highly doubtful in my opinion. Probably really a misplaced one from Peru. I have a specimen from Llallagua that looks like pink mammilary rhodochrosite, but it is really gumucionite.

[al mar]

Thanks Alfredo. The one that I saw from Huanuni could be rhodocrosite but also could be an Mn bearing Carbonate. Regarding ones from Llallagua the aspect was the one that you describe so it would be probably gumucionite.

[Jordi Fabre]

Don't forget that recent theories indicates that color could be not due to the ions and impurities in the average assumed before but mostly by the color centers...
http://www.minsocam.org/msa/collectors_corner/arc/color.htm

If someone here is Master on this field it would be lovely to prepare for us a kind of tutorial or lesson and learn with it more about this thrilling subject.