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Nanoscale Structure of Zoned Laurites from the Ojén Ultramafic Massif, Southern Spain

1
Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, Marti i Franquès s/n, 08028 Barcelona, Spain
2
Institut de Nanociència i Nanotecnologia, IN2UB Facultat de Química, Universitat de Barcelona Av. Diagonal 645, 08028 Barcelona, Spain
3
Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 Cd. de México, Mexico
4
Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain
5
Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-UGR, Avda. de las Palmeras 4, 18100 Armilla, Granada, Spain
6
Centres Científics i Tecnològics, Universitat de Barcelona, Lluís Solé i Sabarís, 1-3, 08028 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(5), 288; https://doi.org/10.3390/min9050288
Received: 17 April 2019 / Revised: 7 May 2019 / Accepted: 8 May 2019 / Published: 11 May 2019
(This article belongs to the Special Issue Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes)
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Abstract

We report the first results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of zoned laurite (RuS2)-erlichmanite (OS2) in mantle-hosted chromitites. These platinum-group minerals form isolated inclusions (<50 µm across) within larger crystals of unaltered chromite form the Ojén ultramafic massif (southern Spain). High-magnification electron microscopy (HMEM), high angle-annular dark field (HAADF) and precession electron diffraction (PED) data revealed that microscale normal zoning in laurite consisting of Os-poor core and Os-rich rims observed by conventional micro-analytical techniques like field emission scanning electron microscope and electron microprobe analysis (FE-SEM and EPMA) exist at the nanoscale approach in single laurite crystals. At the nanoscale, Os poor cores consist of relatively homogenous pure laurite (RuS2) lacking defects in the crystal lattice, whereas the Os-richer rim consists of homogenous laurite matrix hosting fringes (10–20 nm thickness) of almost pure erlichmanite (OsS2). Core-to-rim microscale zoning in laurite reflects a nonequilibrium during laurite crystal growth, which hampered the intra-crystalline diffusion of Os. The origin of zoning in laurite is related to the formation of the chromitites in the Earth’s upper mantle but fast cooling of the chromite-laurite magmatic system associated to fast exhumation of the rocks would prevent the effective dissolution of Os in the laurite even at high temperatures (~1200 °C), allowing the formation/preservation of nanoscale domains of erlichmanite in laurite. Our observation highlights for the first time the importance of nanoscale studies for a better understanding of the genesis of platinum-group minerals in magmatic ore-forming systems. View Full-Text
Keywords: Platinum-group minerals; chromitite; zoned laurite-erlichmanite; nano-mineralogy; focused ion beam; transmission electron microscopy; precession electron diffraction Platinum-group minerals; chromitite; zoned laurite-erlichmanite; nano-mineralogy; focused ion beam; transmission electron microscopy; precession electron diffraction
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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MDPI and ACS Style

Baurier-Aymat, S.; Jiménez-Franco, A.; Roqué-Rosell, J.; González-Jiménez, J.M.; Gervilla, F.; Proenza, J.A.; Mendoza, J.; Nieto, F. Nanoscale Structure of Zoned Laurites from the Ojén Ultramafic Massif, Southern Spain. Minerals 2019, 9, 288.

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