Formation of As(II)-pyrite during experimental replacement of magnetite under hydrothermal conditions Gujie Qian a,b,1 , Joe ¨l Brugger b,c,⇑ , Denis Testemale d,e , William Skinner a , Allan Pring b,c,⇑ a Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia b Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, SA 5000, Australia c Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia d Institut Ne ´el, CNRS et Universite ´ Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France e FAME, ESRF, Polygone Scientifique, 6 rue Jules Horowitz, 38043 Grenoble, France Received 10 April 2012; accepted in revised form 19 September 2012; Available online 28 September 2012 Abstract A ‘new’ type of arsenian pyrite was formed during experimental replacement of magnetite under hydrothermal conditions (T = 125 and 220 °C; P sat ) and in the presence of S(-II) and various As-containing species. The amount of As in pyrite depended on the As-source, with sources containing cationic As (As(II), As(III) and As(V)) resulting in considerably higher amounts of As in the product arsenian pyrite than anionic sources. The highest As content was 23.83 ± 0.20 wt%, correspond- ing to a S:Fe:As molar ratio of 2:0.58:0.42. Electron probe micro-analyses revealed an inverse correlation between the Fe and As contents in the arsenian pyrite, indicating that As is substituting for Fe. Arsenic concentrations were highly inhomoge- neous within the pyrite rim; in general, higher As contents were found within solid pyrite growing on the outer rim, compared to the highly porous and texturally complex pyrite found close to the reaction boundary. This likely reflects different uptake mechanisms for As during the pyrite nucleation and growth stages. X-ray Absorption Near Edge Structure (XANES) analyses showed that the As in the arsenian pyrite was predominantly in the form of As(II). Cross-sectional X-ray photoelectron spec- troscopy (XPS) analysis of the arsenian pyrite confirmed the presence of As(II), but also showed evidence for more oxidized species (As(III) and As(V) oxides), as well as small amounts of polymeric As–As bonding. This indicates a large difference between As in the bulk (XANES measurements) and at the pyrite surface (XPS). Ab initio XANES calculations are consistent with As replacing Fe in pyrite in the form of As(II). Our experimental study suggests that the formal oxidation state of As in this type of arsenian pyrite is close to +2, and that in addition to fluid composition and oxidation state, the reaction path leading to pyrite formation plays a significant role in controlling the chemistry of arsenian pyrite. Ó 2012 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Pyrite (FeS 2 ) is probably the most common sulfide min- eral not only in ore deposits, but also in many sedimentary, magmatic and metamorphic rocks. Pyrite can incorporate large amounts of As (up to 19 wt%, Abraitis et al., 2004; Reich and Becker, 2006 and references therein) as well as minor amounts of other important elements includ- ing Au, Ag, Pb, Co, Ni, Cu, Zn, Hg, W, Bi, and Te (e.g., Cook and Chryssoulis, 1990; Fleet and Mumin, 1997; 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.09.034 ⇑ Corresponding authors. Address: Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Envi- ronmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia. E-mail addresses: Joel.Brugger@adelaide.edu.au, Joel.Brugger @samuseum.sa.gov.au (J. Brugger), Allan.Pring@samuseum.sa. gov.au (A. Pring). 1 Current address: Minerals and Materials Science & Technol- ogy, Mawson Institute, University of South Australia, Adelaide, SA 5095, Australia. www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 100 (2013) 1–10