New Mexico Mineral Symposium — Abstracts

We analyzed four smithsonite (ZnCO₃) samples with electron microprobes in an attempt to decipher the origins of the different colors. We examined samples that were blue green (Kelly mine, New Mexico), yellow (Hanover, New Mexico), blue purple pink with white bands (Sinaloa, Mexico), and green yellow (79 mine, Arizona). Previous authors have proposed, with some geochemical evidence, that the green color of smithsonite is due to copper, the yellow to cadmium, and the blue to cobalt. However, until now, no one has used modern microchemical studies to investigate the chromophores in smithsonite.

The Kelly mine sample exhibits strong color banding that corresponds to a change in solid solution of copper carbonate (CuCO₃) in the smithsonite, with higher CuCO₃ contents (as much as 3.0 wt %) occurring in the strongly colored green bands. No mineral inclusions occur in this sample, and there is little variation of the other minor elements present (calcium and lead). Therefore we believe that copper is the coloring agent in the green smithsonite from the Kelly mine.

"Cadmian" smithsonite from the Hanover mine contains inclusions of pyrite (FeS₂) —300 μm on a side, iron-rich sphalerite or [(Zn,Fe)S], and either hexagonal greenockite or isometric hawleyite (CdS). The CdS occurs in brightly colored bands, which are —10 μm thick and contain about 17 % of these —1 pm diameter inclusions. In addition, as much as 19.5 wt % iron carbonate (FeCO₃) is present in solid solution. From this evidence we hypothesize that both CdS inclusions and iron in solid solution are the coloring agents in yellow smithsonite from this mine.

The blue-purple-pink smithsonite sample from Sinaloa, Mexico, contains copper and cadmium but no cobalt! In addition to our work, others have found that pink "cobaltian" smithsonite lacks cobalt. Thus, the term "cobaltian" smithsonite should be abandoned. Line scans across the color zones show that CuCO₃ is high in the purple-pink regions (1.5 wt %) and even higher (3.0 wt %) in the blue zones. Solid solution cadmium carbonate (CdCO₃) concentrations are fairly uniform from 1.0 to 1.5 wt % in both blue and purple zones of the mineral. The coloring agents in this sample appear to be copper and cadmium but because CdCO₃ is colorless, the cadmium is not coloring the smithsonite directly but it must be altering the way copper colors the mineral. Where the copper concentration is highest, the smithsonite is blue instead of green. As the concentration of copper gets lower, the color goes to purple pink in the presence of cadmium. The white zones are due to abundant inclusions of hemimorphite.

The green smithsonite from the 79 mine contains inclusions of hemimorphite, aurichalcite, a manganese and copper oxide, and CuCO₃ and manganese carbonate (MnCO₃) in solid solution. The inclusions are as much as 1 mm in size and are present mainly at the edges of the material. CuCO₃ and MnCO₃ have the greatest concentrations (as much as 1.7 and 2.0 wt %, respectively) of solid solution impurities and the strongest variation. The copper concentration is not as high as in the blue-green smithsonite from the Kelly material. The combination of lower amounts of copper and the presence of manganese may therefore account for the lighter green and yellow color of this smithsonite sample.

Acknowledgments
Part of this work was supported by the U.S. Department of Energy under Contract DE-AC04- 94AL85000. Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy.
Dr. Nelia Dunbar and Lynn Heizler (New Mexico Bureau of Geology and Mineral Resources) assisted with the electron microprobe analysis. The Cameca SX-100 electron microprobe at the New Mexico Institute of Mining and Technology was partially funded by the National Science Foundation, Grant STI-9413900.