Chemical composition

The chemical formula of the sphalerite is ZnS in its pure form.  It consists of 67% of zinc and 33% of sulphur. Sphalerite is used worldwide for zinc extraction, which in turn is used to make products, chemicals and medicines. Sphalerites usually occurs with some iron replacing the zinc, which can be as high as 36.5% thus its formula can be more accurately described as  (Zn,Fe)S.

Sphalerite with  high iron content is called marmatite. These crystals are black and opaque and cannot be used as gems. Cleofane is colourless or light-coloured (usually yellowish or greenish) sphalerite, it is the most chemically pure variety with low contents of Fe and Mn. Sphalerite often contains minor amounts of other elements such as cadmium, germanium, mercury, and cobalt, that determine the color of the crystals.

The first study on the relation between color and composition of Aliva sphalerites was published in the article of C. Sapalski and F. Gómez in the Bulletin of the Gemological Institute of Spain (nº 34, 1992). The study used electron microprobe analysis on yellow, green and red gem sphalerites from Aliva mine.  The authors stated that red sphalerites have higher contents of Ge and Hg, while green sphalerites have higher amounts of Fe compared to sphalerites whose color are not green.

A Gómez-Fernández et al. study of the chemical composition of the differently colored sphalerite form Cantabria was published in 2000. The study used electron microprobe, XRF, AAS and ICP analyses. The authors concluded that the reddish sphalerite has high contents of Hg, Cu and Cd, the yellowish ones have higher concentrations of Bi, and the green ones are Cu-deficient.

Chemical composition table

Chemical composition of sphalerites from Aliva mine, type II mineralization, responsible for gem-quality sphalerite formation. Extracted from García-Fernández et al., 2000.