COLOR, CRYSTAL-CHEMISTRY, AND MINERAL ASSOCIATION OF A GREEN SPHALERITE FROM STEINPERF, DILL SYNCLINE, FRG

Green sphalerite and the coexisting minerals calcite and prehnite occu r in small veins in a basalt from Steinperf, Dill Syncline, Germany. C hemical analysis by AAS and electron microprobe reveal that all three minerals are almost pure endmembers and contain only minor amounts of the transition elements. It is shown by optical absorption spectroscop y that small amounts of Co2+(80 ppm) in the tetrahedral Zn sites cause the green colour of this particular sphalerite. EPR studies of the sp ecimen reveal that the cubic Zn positions are distorted by the incorpo ration of the Jahn Teller ion Co2+ without affecting the overall cryst al structure. The minerals formed under hydrothermal conditions, start ing with calcite and followed by prehnite, which crystallized under ox idizing conditions as indicated by the incorporation of Fe3+ Sphalerit e formed under more reducing conditions as the last mineral in the seq uence and incorporated all cobalt as Co2+.