The supergene origin of ruthenian hexaferrum in Ni‐laterites

Abstract

For two decades, the nature of Fe-rich, oxygen-bearing, Ru-Os compounds found in the supergene environment has been debated. Ru-Os-Fe-oxides and nano-intergrowths of ruthenium with magnetite have been proposed. We applied FE-SEM, EMPA, l-Raman spectroscopy and synchrotron tts-lXRD to Ru-Os-Fe compounds recovered from Ni-laterites from the Dominican Republic. The results demonstrate that a significant portion of Fe exists in a common structure with the Ru-Os alloy, that is, ruthenian hexaferrum. This mineral occurs both as nanoparticles and as micrometric patches within a matrix of Fe-oxide(s). Our data suggest that supergene ruthenian hexaferrum with a (Ru 0.4 (Os,Ir) 0.1 Fe 0.5 ) Ʃ1.0 stoichiometry represents the most advanced weathering product of primary laurite within Ni-laterites from the Dominican Republic.

Fibrous Platinum-Group Minerals in “Floating Chromitites” from the Loma Larga Ni-Laterite Deposit, Dominican Republic

“…The observation of subsequent Fe-incorporation into secondary Ru-Os-Ir alloys via abiotic and/or biological processes has been used as an evidence for a supergene origin of ruthenian hexaferrum found in Ni-laterites from the Dominican Republic [20]. Here, the presented XRD data of one characteristic fibrous PGM with an isometric morphology suggest that such a PGM transformation from primary laurite toward ruthenian hexaferrum has occurred ( Figure 3A; Tables 1 and 2).…”

“…A comparison with literature data for chemically related mineral phases (Os-group minerals) revealed that the fibrous PGM has rather identical X-ray line intensities and observed d-spacings as ruthenian hexaferrum [20] (Table 1). On this basis, the X-ray powder pattern for the fibrous PGM was indexed, leading to the refined unit-cell dimensions: a 2.6670 (13) Å, c 4.2361 (18) Å, V: 26.095 (27) Å 3 , Z = 2 ( Table 2).…”

“…Recent studies have suggested that these Mg-silicates are not stable at higher levels of the Ni-laterite profile (close to the Mg-discontinuity) and are replaced by secondary Fe oxide(s) [20]. The observation of subsequent Fe-incorporation into secondary Ru-Os-Ir alloys via abiotic and/or biological processes has been used as an evidence for a supergene origin of ruthenian hexaferrum found in Ni-laterites from the Dominican Republic [20].…”

“…Ru-dominated and Fe-rich alloys (e.g., ruthenian hexaferrum) are the most abundant PGM (>90%), followed by lower quantities of Ir-and Pt-bearing Fe-Ni alloys ( [7,20] respectively). Careful examination of all PGM via FE-SEM led to the observation of a rarely occurring (~1%), Ru-dominated subgroup which is characterized by fibrous textures (Figure 2).…”

A review of hexaferrum based on new mineralogical data

“…More recently, there have been a number of studies by Joaquín Proenza and his group at the University of Barcelona on the Loma Peguera and Loma Larga chromitites, in the central part of Loma Caribe peridotite, Cordillera Central of the Dominican Republic (e.g. Proenza et al , 2008; McDonald et al , 2010, Aiglsperger et al , 2017 a , b ). The ultramafic rocks consist mostly of harzburgite, dominantly clinopyroxene-rich harzburgite (modal content up to 5% Cpx) with small amounts of dunite and lherzolite, considered to represent the mantle sequence of an ophiolite (Lewis and Jimenez, 1991; Lewis et al , 2006; Marchesi et al , 2016) and have been heavily serpentinized with a well-developed nickel-rich laterite having formed over them.…”