Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba

José M. González-Jiménez, Artur Deditius, Fernando Gervilla, Martin Reich, Alexandra Suvorova, Malcolm P. Roberts, Josep Roqué, Joaquín A. Proenza

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Abstract

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe)9S8], pyrite (FeS2), and chalcopyrite (CuFeS2). These BMS were found forming composite inclusions (<60 μm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayarí-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt%, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt%. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt% of Ru and <3.5 wt% of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt% and Os <0.2 wt%. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite.

Original languageEnglish
Pages (from-to)1208-1220
Number of pages13
JournalAmerican Mineralogist
Volume103
Issue number8
DOIs
Publication statusPublished - 28 Aug 2018

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Chromite deposits
Cuba
pentlandite
chromites
Sulfides
chromite
base metal
sulfides
platinum
partitioning
Metals
deposits
sulfide
Platinum
platinum group element
metals
nanoparticles
inclusions
Nanoparticles
pyrites

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González-Jiménez, José M. ; Deditius, Artur ; Gervilla, Fernando ; Reich, Martin ; Suvorova, Alexandra ; Roberts, Malcolm P. ; Roqué, Josep ; Proenza, Joaquín A. / Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba. In: American Mineralogist. 2018 ; Vol. 103, No. 8. pp. 1208-1220.
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abstract = "We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe)9S8], pyrite (FeS2), and chalcopyrite (CuFeS2). These BMS were found forming composite inclusions (<60 μm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayar{\'i}-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt{\%}, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt{\%}. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt{\%} of Ru and <3.5 wt{\%} of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt{\%} and Os <0.2 wt{\%}. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite.",
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González-Jiménez, JM, Deditius, A, Gervilla, F, Reich, M, Suvorova, A, Roberts, MP, Roqué, J & Proenza, JA 2018, 'Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba' American Mineralogist, vol. 103, no. 8, pp. 1208-1220. https://doi.org/10.2138/am-2018-6424

Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba. / González-Jiménez, José M.; Deditius, Artur; Gervilla, Fernando; Reich, Martin; Suvorova, Alexandra; Roberts, Malcolm P.; Roqué, Josep; Proenza, Joaquín A.

In: American Mineralogist, Vol. 103, No. 8, 28.08.2018, p. 1208-1220.

Research output: Contribution to journalArticle

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T1 - Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba

AU - González-Jiménez, José M.

AU - Deditius, Artur

AU - Gervilla, Fernando

AU - Reich, Martin

AU - Suvorova, Alexandra

AU - Roberts, Malcolm P.

AU - Roqué, Josep

AU - Proenza, Joaquín A.

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N2 - We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe)9S8], pyrite (FeS2), and chalcopyrite (CuFeS2). These BMS were found forming composite inclusions (<60 μm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayarí-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt%, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt%. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt% of Ru and <3.5 wt% of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt% and Os <0.2 wt%. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite.

AB - We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe)9S8], pyrite (FeS2), and chalcopyrite (CuFeS2). These BMS were found forming composite inclusions (<60 μm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayarí-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt%, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt%. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt% of Ru and <3.5 wt% of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt% and Os <0.2 wt%. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite.

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KW - platinum-group elements (PGE)

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