High-pressure electrical resistivity, Mossbauer, thermal analysis, and micro-Raman spectroscopic investigations on microwave synthesized orthorhombic cubanite (CuFe 2 S 3 ) Usha Chandra a, ⁎, Pooja Sharma a , G. Parthasarathy b a Department of Physics, University of Rajasthan, Jaipur 302 055 India b National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad 500 606 India abstract article info Article history: Received 4 August 2010 Received in revised form 14 October 2010 Accepted 30 January 2011 Available online 4 February 2011 Editor: D.B. Dingwell Keywords: Cubanite Microwave heating Mössbauer spectroscopy High-pressure Electrical resistivity Raman Spectroscopy We present here the powder X-ray diffraction, micro-Raman, thermal, Mössbauer and high-pressure electrical resistivity properties of microwave synthesized orthorhombic cubanite CuFe 2 S 3. The unique physico-chemical conditions required for the formation of orthorhombic cubanite CuFe 2 S 3 prevents its successful synthesis under the normal laboratory conditions. However, in natural occurrence, the mineral occurs in orthorhombic structure along with chalcopyrite (CuFeS 2 ) and pyrrhotite (Fe 1 -x S). A successful attempt to synthesize orthorhombic CuFe 2 S 3 is reported using microwave heating. The DTA and Micro-Raman and Mössbauer spectroscopic measurements on the synthesized sample confirm that the synthetic sample is orthorhombic cubanite. High pressure electrical resistivity measurements on the sample show an irreversible first order phase transformation of orthorhombic cubanite to NiAs structure at ~4 GPa. The high-pressure recovered sample has been characterized by micro-Raman spectroscopy and exhibits Raman bands at 349 and 387 cm -1 , characteristic of isocubanite phase. The present high-pressure and temperature studies on the orthorhombic cubanite may be useful in understanding the thermodynamic behavior of cubanite and its paragenesis in carbonaceous chondrite. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction Determining structure and thermodynamic properties of sulfide minerals is very important to understand the paragenesis of sulfide formation in the earth and planetary system. Sulfide minerals are the most important group of ore minerals, as they provide most of the non ferrous metals (Vaughan, 2006). Sulfide minerals occur in the Earth's mantle and also in Mars's mantle. All the Martian meteorites (Shergotty, Zagami, Nakhla, and Chassigny) have 0.2 wt.% S (King and McLennan, 2010). Vaughan has edited a special volume dedicated to sulfide mineralogy and geochemistry (Vaughan, 2006). Cubanite CuFe 2 S 3 occurs in mineral deposits along with chalcopyrite (CuFeS 2 ) and pyrrhotite (Fe 1 -x S). The detailed experimental study on the thermal phase stability of different mineral assemblages in Cu–Fe–S system has shown that the high-temperature phase of isocubanite transforms to tetragonal cubanite at 252 °C on cooling. The formation temperature of ortho-cubanite (which is the low temperature phase) is not determined. (Yund and Kullerud, 1966; Sugaki et al., 1975). A dense form of cubanite with troilite structure has been synthesized from the tetragonal cubanite at 200 °C and 1 GPa. This phase further converts to the isocubanite on heating above 57 °C at room pressure (Miyamoto et al., 1980), suggesting (i) the difficulties in preparing ortho-cubanite in laboratory and (ii) the isocubanite is the thermodynamically stable phase in the CuFe 2 S 3. Hence there are very little data available about orthorhombic cubanite in the literature. The present study is motivated by the recent findings on the occurrence of orthorhombic cubanite in CI-chondrites, stardust of Comet-Wild 2 and as mineral deposits in chalcopyrite, pyrrhotite matrix suggested their low-temperature formation (below 210 °C) (Berger and Lauretta, 2008; Berger et al., 2009; Macdougall and Kerridge, 1977), which have revived our interest in learning about the origin of orthorhombic cubanite and its thermodynamic phase stability. The crystal structure of orthorhombic CuFe 2 S 3 has an ordered tetrahedral cation sites with S in hexagonal packing. Fe 2+ and Fe 3+ share the adjacent edges of tetrahedra allowing rapid electron exchange between them. Irreversible transformation of low temperature cubanite to high temperature isocubanite at 210 °C is well reported (Miyamoto et al., 1980; Pruseth et al., 1999; Putnis, 1977). The natural orthorhombic cubanite was studied using single crystal X-ray diffraction, Mössbauer spectroscopy, temperature-dependent electrical resistivity, differential thermal analysis (DTA) and micro- Raman spectroscopy (Rosenberg et al., 1997; Dutrizac, 1976 and White, 2009). Similarity between the sulfides — chalcopyrite and cubanite has been studied in details. Sulfur k-edge X-ray absorption spectroscopic studies showed identical spectra indicating similar crystal structure of tetrahedrally-coordinated sulfur and metal atoms (Li et al., 1994). Chemical Geology 284 (2011) 211–216 ⁎ Corresponding author at: High-pressure Physics laboratory, Department of Physics, University of Rajasthan, Jaipur 302055 India. Tel.: +91 141 2711632. E-mail addresses: ushac_jp1@sancharnet.in, chandrausha@hotmail.com (U. Chandra). 0009-2541/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.01.026 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo