Sphalerite

(via Wahroongai News, Volume 30, No 12, December, 1996) Grahame Brown writes:

Sphalerite, otherwise known as zinc blende (Zn, Fe, S), is a cubic mineral that commonly occurs as yellow, brown, orange, green, red, and colorless to gray isometric crystals and cleavage fragments. Due to sphalerite’s high refractive index (2.37-2.43), and a very high dispersion (0.156 or x4 diamond), this principal ore of zinc displays a vitreous to adamantine luster on polished surfaces. Its specific gravity varies from 3.9 to 4.1, depending principally on its iron content. Faceted gems, cut from sphalerite, display many diamond-like characteristics. Consequently, this mineral has been used to imitate diamond—in spite of its ready, perfect dodecahedral (6-directions) cleavage.

The source of some of the world’s finest specimen of this mineral have been the lead zinc deposits of the Picos de Europa Mountains in the Cantabria region of northern Spain. The mines of this area, and their minerals, were described and illustrated by de Baranda & Garcia in the May – June 1996 issue of the Mineralogical Record (pp 177-190).

The mines of this area, particularly the Aliva mines, have been sources of gem quality sphalerite from the 1860s until 1990 (when the last mine ceased operations and was permanently sealed). Aliva mine sphalerite, which occurs with smithsonite, hydrozincite, hemimorphite and other sulphides, such as galena, is found in a carbonate (calcite and dolomite) gangue that fills veins and pockets in carbonaceous limestones that form the mountain/s of the region.

Chemical analysis of this sphalerite have revealed that the causes of its attractive red, yellow and green colors are quite complex. While this sphalerite does contain variable amounts of iron, its various colors also contain variable amounts of other rare elements such as germanium, cadmium, and significant amounts of mercury. The interplay of these elemental impurities in creating various colors is interesting. For example, yellow and green varieties contain little germanium, while green varieties contain highest levels of iron, and red varieties contain highest levels of both cadmium and mercury. Obviously there is a need for additional research before precise cause of color of this sphalerite can be established.

Undoubtedly the best paper written on the very gemmy sphalerites from the Aliva mine is a 1992 paper titled Estudio de la esfalerita la mina de Aliva Santander (Espana) by Cristina Sapalski & Fernando Gomez that was published in the June issue of Boletin del Insituto Gemologico Espanol.