Crystal structures of rhodium-containing erlichmanite–laurite solid solutions (Os_1–x–yRu_xRh_yS₂: x = 0.09–0.60, y = 0.07–0.10) with unique compositional dependence

Abstract

Rh-rich and Ir-poor erlichmanite-laurite OsS2-RuS2 solid solutions have been discovered at placers in Haraigawa, Misato-machi, Kumamoto, Japan. Microprobe analysis was performed to identify solid solutions containing few sub-components other than Rh. Approximately 10 at% Rh was found to be present in the solid solutions. Structural refinement was performed using four natural samples: Os0.32Ru0.61Rh0.07S2, Os0.49Ru0.43Rh0.08S2, Os0.58Ru0.33Rh0.08S2 and Os0.81Ru0.09Rh0.10S2. The unit-cell parameters for the solid solutions containing Rh from Haraigawa varied from 5.61826(6) to 5.63142(8) Å. The (Os, Ru, Rh)-S distances in the Os1-x-yRuxRhyS2 system were almost constant with a small variation of 0.001 Å. Conversely, the S-S distances varied significantly, with variations approaching 0.1 Å. Rh substitution of Os rather than Ru showed a larger impact on the crystal structure. The atomic displacement ellipsoid of both cations and anions was almost spherical, and no elongation along the M-S and S-S bond directions was observed. The bulk Debye temperatures were estimated from the Debye-Waller factor for the sulfide site. The bulk Debye temperatures of pure OsS2 and RuS2 were 688 K and 661 K, respectively, which suggests that the melting point of erlichmanite is

Micron-to-nanoscale investigation of Cu-Fe-Ni sulfide inclusions within laurite (Ru, Os)S2 from chromitites

“…We suggest that in our case, zonation took place by way of a complete mutual substitution of Ru⇔Os, very likely resolved in the structures via enlargement of the lattice parameters (Supplementary Information ESM_2, ESM_3, ESM_4 and ESM_5). Differences in the lattice parameters between laurite (RuS 2 ) and erlichmanite (OsS 2 ) have previously been reported for both synthetic and natural origin, and interpreted as a result of higher atomic/ionic radii of Os than Ru (Kitahara et al 2023). Nevertheless, the range of variation in our case study (see Supplementary Information ESM_2 and ESM_5) is wider than reported previously for the Pa3 structure these Ru-Os sulfides (5.6095-5.63142 Å;Sutarno et al 1967;Lutz et al 1990;Kitahara et al 2023), suggesting that the small amounts of Ir and Rh detected in these laurite grains could also impact the width of the lattice parameters.…”

“…Differences in the lattice parameters between laurite (RuS 2 ) and erlichmanite (OsS 2 ) have previously been reported for both synthetic and natural origin, and interpreted as a result of higher atomic/ionic radii of Os than Ru (Kitahara et al 2023). Nevertheless, the range of variation in our case study (see Supplementary Information ESM_2 and ESM_5) is wider than reported previously for the Pa3 structure these Ru-Os sulfides (5.6095-5.63142 Å;Sutarno et al 1967;Lutz et al 1990;Kitahara et al 2023), suggesting that the small amounts of Ir and Rh detected in these laurite grains could also impact the width of the lattice parameters. Moreover, in the laurites studied here, a complete homogenization of Os within the nominal RuS 2 laurite matrix was very likely favored by a slower rate of undercooling of the chromitite volatile component accompanying the sulfide melts (Mungall et al 2015;Blanks et al 2020;Barnes et al 2023).…”

“…The first Information ESM_3 and ESM_5). works indicate that pure RuS 2 laurite crystallizes from S-undersaturated basaltic melt at 1200-1300 ºC and log fS 2 from − 2 to -1.3, with high Os solubility dependent on decreasing temperature (Brenan and Andrews 2001;Andrews and Brenan 2002b) and, above all, increasing fS 2 (Kitahara et al 2023). Therefore, Os-rich laurite in both Taitao and Kevitsa should have crystallized at a slightly lower temperature (∼1,150-1200 ºC) but higher fS 2 than pure laurite synthesized in experimental works.…”

“…is common to three octahedra (Müller 2013). This does not only allow a complete solid-solution series between the two end-members Ru 2+ and Os 2+ , but also noticeable substitution (in the order of a few wt%) of other PGEs (Ir, Rh, Pd and Pt) and the transition metals Cu, Fe and Ni (Cabri 2002;González-Jiménez et al 2009;Sinyakova et al 2022;Kitahara et al 2023). Therefore, at first glance, the cubic structure of laurite should be able to dissolve the amounts of Cu, Fe, Ni, and S necessary to exsolve discrete Cu-Fe-Ni sulfides upon cooling.…”