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Fernando Gervilla, University of Granada (Spain)
Jose Marķa Gonzalez-Jimenez, University of Granada (Spain)
Thomas Kerestedjian, Bulgarian Academy of Sciences (Bulgaria)
Joaquin Antonio Proenza, University of Barcelona (Spain)
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Ruthenian pentlandite [(Ni,Fe)8RuS8; name not approved by IMA as a new mineral] is a minor, but relatively frequent mineral in ophiolite chromitites. It always occurs in multiphase aggregates with laurite or secondary Ni-Fe alloys, which replace pentlandite. These polyphase grains are found in ferrian chromite margins of chromite, in fractures of chromite, associated with secondary, hydrous silicates and/or in the interstitial serpentinised silicate matrix between chromite grains.
The study performed on the metamorphosed chromitites of the Dobromirtsi Massif (SE Bulgaria) confirms the described textural relationships of Ru-pentlandite bearing PGM aggregates, occurring in ophiolite chromitites. At Dobromirtsi these aggregates only occur in the altered, ferrian chromite margins of chromite grains and consist of Ru-pentlandite (with 5.47-7.87 at.% Ru) associated with laurite, minute grains of irarsite and/or Os-Ir alloys. The latter occur at the contact between Ru-pentlandite and laurite or form minute, irregularly distributed inclusions in Ru-pentlandite. The contact between Ru-pentlandite and laurite is very irregular, often showing corrosion gulfs, penetrating in laurite. Some isolated Ru-pentlandite grains (containing Os-Ir alloys) have also been observed. In one aggregate included in ferrian chromite, laurite is associated with a grain of a Ru-rich (up to 14.76 at.% Ru), monosulphide solid solution (mss)-type phase. This phase, like Ru pentlandite, contains some minute inclusions of Os-Ir alloys and shows an irregular contact with laurite.
The fact that Ru-pentlandite always occurs in altered chromite or associated with the serpentinised silicate matrix of chromitites, suggests that it cannot be a part of the primary assemblage. Its position rather proves that it is formed during the alteration process. Furthermore, the geometry of the contact between Ru-pentlandite and laurite suggests that the former could form at the expense of the latter. This replacement needs a partial loss of sulphur and a significant supply of Fe and Ni. This ion exchange can take place during serpentinisation, since alteration of olivine to serpentine produces significant amounts of H2, which lowers both fO2 and fS2 by its reaction with O2 - to form water and with S2 - to form H2S. As a consequence, the equilibrium of sulphides with the generated H2-rich fluids gives rise to sulphides progressively impoverished in sulphur. Serpentinisation also releases Fe and Ni from olivine. In this environment laurite can react with fluids - to form first the Ru-rich, mss-type phase and afterwards - Ru pentlandite. This reaction releases Os and Ir from laurite to form Os-Ir alloys. The described Ru-pentlandite forming reaction most probably takes place at relatively high temperature (∼300-500°C). At lower temperature, desulphurization of laurite produces Ru-Os-Ir-Fe-Ni alloys, a process widely described in the literature.
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