Coexisting nickel diarsenide polymorphs in the five-element mineralization of the Bihor metallogenetic district, Apuseni Mountains, Romania

The three known polymorphs of nickel diarsenide were found to coexist in apparent equilibrium in the five-element mineralizations of the Bihor Metallogenetic District. It is the first identification of pararammelsbergite and krutovite in Romania, and the locality joins a very limited list of occurrences where all three NiAs2 polymorphs coexist. Both phases newly identified in Romania are easily distinguishable from rammelsbergite by their optical properties and display consistent compositional differences manifested in stoichiometry and degree of isomorphic substitution. The NiAs2 polymorphs coexist with nickeline and skutterudite and apparently predate cobaltite-gersdorffite in a mesothermal depositional sequence characterized by initial separation of Ni and Co in metal-poorer arsenides, followed by continuous decrease of arsenic activity, increase in sulfur activity, modal increase of nickeline and Co enrichment in the gersdorffite-cobaltite series.