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Coloring agent in pink smithsonite from Choix

Posted by NH  
NH March 08, 2009 02:05AM
I am wondering what is responsible for the color of the pink smithsonite from Choix, Sinaloa, Mexico. Cobalt? Manganese? Presumably the purplish ones contain a combination of the pink coloring agent and copper..?

Howard Heitner March 08, 2009 04:41AM
Pink smithsonite from Tsumeb is colored by cobalt, I presume it is the same for the Mexican material.
Mark Heintzelman March 08, 2009 07:34AM
google the report THE COLORS OF SMITHSONITE: A MICROCHEMICAL INVESTIGATION: Patricia L. Frisch and Virgil W. Lueth, New Mexico Bureau of Geology & Mineral Resources, Socorro, NM
Robert Meyer March 08, 2009 07:37AM
The pink smithsonite I have checked from Tsumeb was not cobaltian, despite the commonly held belief that cobalt is the cause of the pinkness. I am sure that some is probably cobaltian, but all of the pieces I have checked were either manganoan or ferroan or both. They were really pink, too.

Thus, I would not assume it is cobalt at Choix that causes the pink color.

Mark Heintzelman March 08, 2009 07:52AM
It is proposed in the above mentioned report that the color is due to the combined effect of copper carbonate and cadmium carbonate (no cobalt present) with copper as the chromophore, cadmium affecting the range of color expression. The higher concentration of copper the bluer the color, with less copper the color ranges from violet to pink.


Edited 1 time(s). Last edit at 03/08/2009 07:57AM by Mark Heintzelman.
NH March 08, 2009 08:16AM
Thanks for all the help so far.

Mark: I looked up your source and found the abstract. The authors claim that Cu and Cd in solid solution are responsible for the pink (low Cu) and blue (higher Cu) colors, which is pretty surprising. They state that the Cd must be changing the way that the Cu colors the mineral; anyone have any guesses how?
Knut Eldjarn March 08, 2009 09:28AM
I am no expert in the field but can try to give you the same explanation I used to give my chemistry students many years ago.The perceived colour of a material is a function of the composition of the light (wavelenghts) shining on or entering the material and the light (wavelenghts) reflected, absorbed or emitted by the material. How a material absorbs or reflects photons is a result of the interaction between the electrons orbiting the molecules and the wavelengths and energy in the light hitting the orbiting electrons. I doubt that anyone has studied the specific properties of the electrons orbiting the Smithsonite-molecules and thus any explanation would have to be a general one. Introducing atoms that influence the orbiting erlectrons could potentially change the colour of the object. Any atoms with "reactive" electrons - i.e. metals or halogenides would be expected to have a high potential to influence the orbiting electrons and maybe change the colour of a material. Inert gases and stable compunds would not be expected to have such effect. Cu and Cd have different properties related to their electronegativity and how their electrons react with other atoms and molecules. It is no wonder if by a coincidence in nature the presence of different amounts of these potential chromophores could have the effect on the colour of Smithsonites as descibed in the paper refered to in the thread. But even if the paper documents a correlation between the amounts of Cu and Cd and the colour changes, there may very well be unknown factors X and Y at work also.
Maybe this explanation does not answer your question, but at least it is a simplistic approach to understanding the mechanisms causing the great variety of colours in nature.

Edited 2 time(s). Last edit at 03/08/2009 07:52PM by Knut Eldjarn.
Robert Meyer March 08, 2009 08:48PM
Although the authors of the abstract Mark pointed out did analyze specimens from Sinaloa, I don't think I would make the assumption that copper and cadmium were the chromophores for all pink smithsonite. As I mentioned, I have analyzed a number of specimens of pink smithsonite from Tsumeb using the same methods as the authors of the abstract. All of the samples had either managanese, iron, or both present. None had appreciable amounts of copper, cadmium, or cobalt.
Howard Heitner March 09, 2009 08:15PM
As a (now retired) scientist I was always taught to review the literature before embarking on a research project. During my career, I have encountered a few instances where the literature was completely wrong. After several people disagreed with my comments about “cobaltian pink smithsonite” I went to my library to find a reference on the various colors of smithsonite. There in “A Field guide to Rocks and Minerals “ by the late Dr. Fred Pough I saw that, in his description of smithsonite physical properties. he lists the colors “White, yellow (from Cd) greenish or bluish (from Cu) or pink (from Co)”. I also found in “Field Guide to Rocks and Minerals of Southern Africa” by Bruce Cairncross that the pink smithsonite is cobaltian or manganoan. I also see three pages of pictures of “cobaltian smithsonite” on as well numerous pictures on dealers websites etc.

So, there is no shortage of opinions. What seems to be lacking is data. The color of a transition metal ion in solution is caused by electronic transitions between unfilled d orbitals. These energy levels are affected by the oxidation state of the atom and what other atoms are around it (the ligand or crystal field). In a mineral, a transition metal impurity ion is essentially “dissolved” in the mineral crystal, substituting for one of the component elements of the mineral. To really prove that an impurity ion causes a particular color in a mineral, first it has to be shown that the ion is present and second that it is dispersed in the crystal structure and is in the right oxidation state. The first part is easy, the second part is not so easy. Any analytical method in which the mineral is completely dissolved is suspect, because any trace components found in the solution might be from inclusions and the oxidation state might change during sample preparation. There is a way around this problem however. If synthetic crystals of the mineral can be grown containing only one trace element, the absorption spectrum ( which is seen by the human eye as color) could be compared to the natural mineral. A good example of this is synthetic ruby, which is pure corundum doped with chromium. It has the same or similar color to the natural mineral. (and is sometimes sold as natural ruby by dishonest people).

So, now is the time for everyone to send in whatever data you have and how you reached your conclusions. Perhaps a prize should be offered for the first synthetic pink smithsonite crystal.
NH March 10, 2009 07:11AM
"The color of a transition metal ion in solution is caused by electronic transitions between unfilled d orbitals. These energy levels are affected by the oxidation state of the atom and what other atoms are around it (the ligand or crystal field)."

This is the main reason that I'm confused by the role of Cd, if any, in producing colors: Cd2+ has all d orbitals filled (CdS has a low band gap and absorbs light by transitions between its valence and conduction bands). Correlation doesn't mean causation, and as Knut suggested there might be other factors at work here...

Synthetic smithsonite would certainly be interesting.
Howard Heitner March 10, 2009 02:03PM
The color of yellow smithsonite is attributed to inclusions of greenockite rather than substitution of Cd in the smithsonite. See for example
Ralph Bottrill February 19, 2012 01:44AM
I updated the description of cobaltoan smithsonite based on Bob Meyer's studies, but if anyone can verify significant Co in smithsonite it would be nice to know?

Ralph Bottrill February 26, 2012 06:57AM
We have one confirmed cobaltian smithsonite: Vignola Mine, Vignola-Falesina, Valsugana, Trento Province, Trentino-Alto Adige, Italy. Giorgio Bortolozzi reports: "cobaltoan smithsonite contained Co < 1% from EDX performed in the seventies by different M.Boscardin,G.Comelet, Michele,R.Marsetti..."

Any others?.

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