Colour, crystal chemistry, and mineral association of a green sphalerite from Steinperf, Dill Syncline, FRG

Green sphalerite associated with calcite and prehnite occurs in small veins in a basalt near Steinperf, in the Dill syncline, on the SE border of the Rhenish Massif, Germany. AAS and EPMA results show that all three minerals are of virtually pure end-member composition, and contain only minor amounts of the transition elements. It is shown by optical absorption spectroscopy that small amounts of Co (super 2+) (80 ppm) in the tetrahedral Zn sites cause the green colour. The cubic Zn positions are shown by e.p.r. to be distorted by the incorporation of the Jahn-Teller ion Co (super 2+) without affecting the overall crystal structure. Calcite and prehnite crystallized under oxidizing conditions, as indicated by the incorporation of Fe (super 3+) in the prehnite, whereas sphalerite formed later under reducing conditions and incorporated all cobalt as Co (super 2+) .