First occurrences of Ni-phosphides in chromitites from the ophiolite complexes of Alapaevsk, Russia and Gerakini-Ormylia, Greece

摘要: Small grains, approximately 15 μm in size, of a Ni-phosphide, have been discovered two chromite deposits associated with altered mantle peridotite: (i) Bakanov Kluch, Alapaesk ophiolite, Russia and (ii) Agios Dimitrios, Gerakini-Ormylia Greece. On the basis composition, both Kluch Dimitrios chromitites are classified as Cr-rich boninitic affinity. Under reflected-light optical microscope, Ni-phosphides yellow colour isotropic. Their calculated stoichiometry, based on average 20 electron microprobe analyses, approaches ideal formula (Ni,Fe)5P. One essential condition for formation terrestrial minerals Ni-Fe-P system is presence highly reducing local geochemical environment. Serpentinization oceanic peridotites produces fluids containing dissolved H2 resulting from reduction H2O. Therefore, we suggest that (Ni,Fe)5P Alapaevsk were very likely formed after precipitation host during serpentinization process at low temperatures. Nickel may released alteration olivine phosphorous represent an product apatite. The grains analyzed present work contain highest value Ni they characterized by to P ratio reported so far similar minerals. stoichiometry indicates found new mineral species. However, their small size prevents acquisition crystallographic data support this assumption.