We report the first occurrence of cobalt pentlandite in oceanic deposits at 14DG45'N, Mid-Atlantic Ridge (MAR). The mineral occurs as small (less than 20 mu m), round to irregular grains and aggregates associated with chalcopyrite, bornite, isocubanite, chalcocite, digenite, with minor pyrite and marcasite, and with rare covellite and Au-bearing zincian copper. On the basis of twenty-two analyses, the compositional range (in wt.%) is: Co 42.9-53.9, Ni 7.2-12.4, Fe 3.4-5.5, Cu 0.7-9.4 and S 31.6-32.9. Seventeen of the twenty-two compositions plot between Me 9 S 8 and Me 15 S 13 . The narrow range of sulfur contents corresponds closely with those of synthetic pentlandite (44.2-47.9 at.%). The cobalt content of a pentlandite-group mineral depends on the mineral assemblage with which it is associated.

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