Mineralogical Magazine, 2015, Vol. 79(2), pp. 377–385

1 1 2 2 3 3 4 Natural occurrence of monoclinic Fe S nano-precipitates 4 5 3 4 5 6 in pyrrhotite from the Sudbury ore deposit: a Z-contrast 6 7 7 8 imaging and density functional theory study 8 9 9 10 10 11 HUIFANG XU*, ZHIZHANG SHEN AND HIROMI KONISHI 11 12 12 13 NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton Street, 13 14 Madison, WI 53706, USA 14 15 [Received 26 March 2014; Accepted 30 August 2014; Associate Editor: M. Welch] 15 16 16 17 17 ABSTRACT 18 18 19 19 A monoclinic form of Fe3S4, a polymorph of cubic greigite, occurs as a precipitate in samples of 20 pyrrhotite collected from the Sudbury ore deposit. The nano-crystal precipitates are in a topotaxial 20 21 21 relationship with the host pyrrhotite-4C (Fe7S8). The precipitate and the host pyrrhotite have a coherent 22 (001) interface. Half of the octahedral layers in the crystal structure are fully occupied by Fe, while the 22 23 other half of the octahedral layers are occupied byFeatomsandvacanciesinanorderedmanneralong 23 24 24 the a axis. The crystal structure of the Fe3S4 nano-precipitates has monoclinic symmetry with space 25 group of I2/m.Itsc dimension is 6% smaller than that of the host pyrrhotite due to the large number of 25 26 vacancies in the structure. Fractional coordinates for S and Fe atoms within the unit cell are determined 26 27 from Z-contrast images and density functional theory (DFT). The calculated results match the 27 28 28 measured values very well. It is proposed that the monoclinic Fe3S4 nano-precipitates formed through 29 ordering of vacancies in pyrrhotite with a low Fe/S ratio (i.e. <0.875) at low temperature. 29 30 30 31 31

32 KEYWORDS: iron sulfide, greigite, pyrrhotite, monoclinic Fe3S4, Sudbury, Z-contrast imaging, density 32 33 functional theory, vacancy ordering, aberration-corrected scanning transmission electron microscopy, 33 34 magnetic mineral. 34 35 35 36 Introduction 36 37 of brezinaite, Cr3S4 (Fleet, 1982). Here we report 37 38 MAGNETIC Fe sulfide with the Fe3S4 stoichiometry an Fe sulfide with large numbers of vacancies at 38 39 generally occurs as greigite with the inverse spinel octahedral sites based on Z-contrast imaging. Both 39 40 structure (Makovicky, 2006). A monoclinic form numbers of vacancies and ordering of vacancies 40 41 of Fe sulfide was reported in a synthetic Fe-sulfide will affect the transport properties of minerals 41 42 product containing pyrrhotite, smythite and mono- (Langenhorst et al., 2014). Monoclinic Fe3S4 may 42 43 clinic Fe3S4 (Fleet, 1982). Smythite with stoichio- occur as Fe sulfide nano-crystals or as a precursor 43 44 metry of Fe9S11 contains more vacancies than that of Fe-sulfide minerals like marcasite and pyrite 44 45 of pyrrhotite, Fe7S8, and is characterized with a based on the ability of pyrrhotite to transform into 45 46 lower proportion of vacancies than Fe3S4 (Taylor marcasite (Fleet, 1978; Murowchick and Barnes, 46 47 and William, 1972). Smythite is not a polymorph 1986; Posfai and Buseck, 1997). Because mono- 47 48 of greigite (Taylor and William, 1972; clinic Fe3S4 is also magnetic, it has the potential to 48 49 Makovicky, 2006). It was proposed that mono- be a hard magnetic material with large magnetic 49 50 clinic Fe3S4 has a crystal structure similar to that coercivity, such as in monoclinic Fe3Se4 (Long et 50 51 al., 2011). 51 52 Although conventional high-resolution trans- 52 53 * E-mail: [email protected] mission electron microscopy (HRTEM) can 53 54 DOI: reveal superstructures very well, it is difficult to 54

# 2015 The Mineralogical Society HUIFANG XU ET AL.

1 locate exact positions of atoms and vacancies in scopy (STEM) analyses were carried out using a 1 2 pyrrhotite using this method due to multiple FEI Titan 80-200 aberration-corrected scanning 2 3 diffraction effects in phase-contrast imaging transmission electron microscope operated at 200 3 4 (Pierce and Buseck, 1974; Posfai and Buseck, kV. This microscope is equipped with a CEOS 4 5 1997). However, Z-contrast imaging that uses probe aberration corrector, an EDAX high- 5 6 elastically scattered, high-angle, non-coherent resolution X-ray energy-dispersive spectroscopy 6 7 electrons can directly locate positions of atoms (EDS) detector and a Gatan image-filtering 7 8 and vacancies along the beam direction system. Probe current was set at 24.5 pA. All 8 9 (Pennycook, 2002; Kirkland, 1998; Xu et al., Z-contrast images were acquired using a camera 9 10 2014). Z-contrast imaging is a low electron-dose length of 128 mm. Signal intensity is proportional 10 1.97 11 imaging method compared to normal HRTEM to atomic number (~Z for the experiment 11 12 imaging and as such can avoid electron beam- condition using a 128 mm camera length) and the 12 13 induced changes, such as vacancy disordering or number of atoms along the beam direction for the 13 14 re-ordering in the specimen during image image acquisition condition (Shi, 2013; Xu et al., 14 15 acquisition. 2014). High angle, annular dark field (HAADF) 15 16 STEM imaging (or Z-contrast imaging) is capable 16 17 of a spatial resolution <0.1 nm using the 17 Sample and experiment 18 aberration-corrected STEM. The STEM sample 18 19 The pyrrhotite sample was collected from the was prepared by crushing selected pyrrhotite 19 20 Sudbury ore deposit in the Sudbury District, fragments between two glass slides with ethanol. 20 21 Ontario. The specimen (No. 2-102) is one of the A drop of the suspension was placed on a lacy 21 22 teaching samples in the Department of carbon-coated Cu grid and air dried. The probe 22 23 Geoscience, University of Wisconsin-Madison aberration correction was carried out first using a 23 24 collection. Sudbury ore mineralogy and petrology standard sample of nano-gold particles. The 24 25 are well documented (Hawley, 1962; Naldrett et specimen was lightly plasma cleaned before 25 26 al., 1970). Scanning transmission electron micro- inserting it into the STEM column. 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 FIG. 1. Z-contrast images from areas of pyrrhotite-4C with stoichiometry of Fe7S8 along the [010] direction (a) and 48 [1¯10] directions (b). Bright spots represent positions of octahedral columns without vacancies. Projections of atoms 48 49 based on a pyrrhotite-4C structure are also overlaid on the images. Small yellow dots are S atoms, large orange dots 49 50 are Fe atoms in octahedral columns without vacancies and small orange dots are columns with alternating Fe and 50 51 vacancy sites along the beam direction. Note the difference in sequences of Fe atoms (orange dots) along c: big-big- 51 52 big-small-big-big-big-small...(corresponding to strong-strong-strong-weak-strong-strong -strong-weak- ... in the 52 53 image) for the [010] direction image (a) and big-big-big-big-big-small-big-small (corresponding to strong-strong- 53 54 strong-strong-strong-weak-strong-weak- ...) for the [1¯10] direction image (b). 54

378 Z-CONTRAST IMAGING AND DFT STUDY OF NATURAL MONOCLINIC Fe3S4

1 Density functional theory (DFT) calculations Results and discussion 1 2 were carried out using Vienna ab initio simulation 2 3 code (VASP) (Kresse et al., 1996). The general The Z-contrast images from pyrrhotite areas show 3 4 gradient approximation (GGA) with Perdew, layers of octahedra parallel to (001). Bright spots 4 5 Burke and Ernzerhof (PBE) parameters was in the images represent columns of octahedral 5 6 employed (Perdew et al., 1996). The projector- sites occupied fully by Fe. The columns of 6 7 augmented wave (PAW) method with an energy octahedrally coordinated sites with both Fe 7 8 cutoff of 420 eV was used. K meshes of 26261 atoms and vacancies are less bright in the image 8 9 were found to be within convergence criteria for (Fig. 1). Columns of S atoms are located between 9 10 monoclinic Fe3S4. In order to take into account the octahedral layer lacking vacancies and the 10 11 the on-site Coulomb repulsion of 3d electrons in octahedral layer with vacancies (Fig. 1). The 11 12 Fe atoms, a simplified (rotationally invariant) projection of a pyrrhotite structure model from 12 13 approach to GGA+U (Dudarev et al., 1998) was Takonami et al. (1972) is also overlaid on the 13 14 carried out. In Dudarev’s method, an effective U images. Yellow dots represent columns of S 14 15 parameter, Ueff =UÀJ, is used. According to atoms. Small and large orange dots represent Fe 15 16 previous DFT calculations of cubic Fe3S4 (Roldan columns with and without vacancies, respectively. 16 17 et al., 2013), U = 1 eV is a reasonable value where It is common to find neighbouring pyrrhotite 17 18 calculated lattice parameters and band gap agree domains sharing one (001) plane and off by 120º 18 19 with experimental data. The initial magnetic around the normal of (001), i.e. the b axis of the 19 20 ordering was set to be high-spin ferromagnetic. top domain is parallel to [1¯1¯0] of the bottom 20 21 The initial structures were calculated fully in both domain as shown in Fig. 2. The neighbouring 21 22 relaxation conditions, where cell shape and domains across the (001) interface (arrowed) are 22 23 volume are allowed to relax, and constrained related by 120º rotations. The orientation 23 24 conditions, where cell shape and volume are difference is a result of ordering of vacancies 24 25 fixed. from high temperature disordered hexagonal 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 FIG. 2. Z-contrast image (a) and corresponding noise-filtered image (b) showing a stacking fault between the 50 51 neighbouring pyrrhotite-4C domains with 120º angle difference around the normal of (001). Arrows indicate the 51 52 interface between the neighbouring domains. The slight distortion of the image resulted from the specimen drifting 52 53 during the image acquisition. FFT patterns from the top domain and lower domain show the [010] zone axis and the 53 54 [1¯1¯0] zone axis for neighbouring domains (c, d), respectively. 54

379 HUIFANG XU ET AL.

1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 FIG.3.A[1¯1¯0] zone axis selected area electron diffraction (SAED) pattern from pyrrhotite (a); SAED pattern from 14 14 an area with neighbouring pyrrhotite domains related by 120º rotation around the normal of (001). (b). SAED pattern 15 15 superimposed on diffraction patterns from the [010] zone axis and [1¯1¯0] zone axis patterns. The patterns have 00l 16 16 diffraction spot positions. There is a small angle difference between (400)* and (2¯20)* for the twinned domains (b). 17 The neighbouring pyrrhotite domains are related by 120º rotation around the normal of (001). The ordering of 17 18 vacancies results in domains with different orientations. Spots of 002, 006 and other 00l (l = 4n) reflections result 18 19 from multiple diffraction. 19 20 20 21 21 22 pyrrhotite to pseudo-hexagonal pyrrhotite-4C. also be very close to half of pyrrhotite’s b value. 22 23 The noise-filtered image for this case better The unit-cell parameters for the nano-precipitate 23 24 illustrates the positions of columns of S atoms, are a = 5.94, b = 3.43, c = 10.70 A˚ , b = 91.0º. 24 25 Fe sites (with and without vacancies) and the Reported unit-cell parameters for a synthetic 25 26 interface between the neighbouring pyrrhotite monoclinic Fe3S4 intergrown with smythite and a 26 27 domains (Fig. 2). Selected-area electron diffrac- host pyrrhotite (Fleet, 1982) are a = 5.98, b = 27 28 tion patterns from a single domain and overlapped 3.42, c = 10.64 A˚ , b = 91.9º. 28 29 domains are illustrated in Fig. 3. The diffraction High-magnification Z-contrast imagery and the 29 30 spot labelled 400 in Fig. 3 results from a [010] corresponding noise-filtered image clearly show 30 31 zone axis domain and is very close to the the positions of columns of Fe atoms without 31 32 diffraction spot labelled 2¯20, which results from 32 33 a[1¯1¯0] zone axis domain. Areas with stacking 33 34 disorder also occur locally. The fast Fourier 34 35 transform (FFT) pattern from the stacking- 35 36 disordered area shows streaking of 20l and 1¯1l 36 37 along the c* direction. 37 38 The pyrrhotite sample studied here contains 38 39 precipitates with more vacancies than the host 39 40 pyrrhotite-4C,Fe7S8 (Fig. 4). Careful examination 40 41 of the image indicates that one quarter of the 41 42 octahedral sites are occupied fully by vacancies 42 43 (Fig. 4). A FFT pattern taken from the image 43 44 indicates that the precipitate has a smaller c 44 45 dimension than that of the host pyrrhotite-4C, 45 46 although their a dimensions are very similar 46 47 (Fig. 5). Using pyrrhotite-4C as a reference, the 47 48 measured c dimension for monoclinic Fe3S4 is 48 49 ~94% of that of the host pyrrhotite. Its c* 49 50 directions are rotated ~0.5º relative to that of the 50 51 host pyrrhotite (Fig. 5). The precipitate’s a value 51 52 is very close to half of pyrrhotite’s a value due to a FIG. 4. A Z-contrast image showing the monoclinic 52 53 coherent interface between the host and precipi- Fe3S4 precipitate in coherent relationship with host 53 54 tate. Therefore, the precipitate’s b value should pyrrhotite (lower left corner). 54

380 Z-CONTRAST IMAGING AND DFT STUDY OF NATURAL MONOCLINIC Fe3S4

1 vacancies (bright spots), S columns (less bright) 1 2 and vacancy-bearing columns (dark areas between 2 3 the bright spots; Fig. 6). Positions of Fe atoms in 3 4 the layers of octahedra without vacancies are 4 5 slightly off the (001) plane in both directions. 5 6 Face-sharing of neighbouring Fe octahedra results 6 7 in a slight shift of Fe atoms towards adjacent 7 8 vacant sites (Fig. 6). Outlines of the representa- 8 9 tive unit cells are illustrated in Fig. 6 by dashed 9 10 lines. Fe(1) atoms lie at the origin and centre of 10 11 the unit cell. Fe(2) atoms located in vacancy-free 11 12 layers are displaced from centres of octahedra 12 13 (Fig. 6b). High concentrations of vacancies in the 13 14 precipitate results in a shrinkage of the unit cell 14 15 along c. The stoichiometry of the precipitate is 15 16 Fe3S4, based on distributions of Fe columns and 16 17 vacancy columns in the image presented in Fig. 6. 17 18 X-ray EDS spectra indicate that the precipitates 18 19 have a lower S/Fe ratio than the host pyrrhotite. 19 20 Using EDS spectra from host pyrrhotite of 20 21 stoichiometry Fe7S8 as a standard, the average 21 22 FIG. 5. An FFT pattern from an image showing the Fe/S ratio for the precipitate is 0.73, which is very 22 splitting of 008 spots (from the monoclinic Fe3S4) and a 23 close to an ideal stoichiometry of Fe3S4. The 23 0016 [overbar? spot of the host pyrrhotite-4C. 24 sulfide-based ores in the Sudbury structure are 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 50 51 FIG. 6. High-magnification Z-contrast image (a) and noise-filtered image (b) along b showing positions of Fe 51 52 columns, S columns and vacancy columns. Projections of atoms based on the proposed structure model are also 52 53 overlaid on the images. Intensity profiles across Fe2 atoms and Fe1 atoms are illustrated in (c) and (d) respectively. 53 54 The vacancy sites between neighbouring Fe1 atoms are obvious because of low signal intensity. 54

381 HUIFANG XU ET AL.

1 related to impact melt (Keays and Lightfoot, monoclinic Fe3S4 is ferrimagnetic (Andresen and 1 2 2004). The slightly low Fe/S ratio of the Laar, 1970). The calculated structure has unit-cell 2 3 pyrrhotite may be related to impact melt parameters a = 5.9013, b = 3.4267, c = 11.0511 A˚ , 3 4 generated from sedimentary rocks and sulfur in b = 93.1º. Fractional coordinates for these atoms 4 5 a less reducing environment. are also listed in Table 1. The host pyrrhotite with 5 6 a coherent (001) interface will constrain the unit 6 7 cell of the precipitate in a and b directions. 7 Structure determination 8 Therefore, the a and b dimensions of nano- 8 9 The ideal structure for monoclinic Fe3S4 with the precipitates are slightly larger than those of the 9 10 brezinaite structure (monoclinic Cr3S4) can be calculated ideal unit cell. The structure for 10 11 obtained based on ideal hexagonal packing of S monoclinic Fe3S4 was also calculated using the 11 12 atoms. Fractional coordinates for Fe and S in the measured unit-cell parameters (Table 1). The 12 13 ideal hexagonal packing structure for a mono- calculated fractional coordinates are consistent 13 14 clinic unit cell are listed in Table 1. Because with the values measured from Z-contrast 14 15 fractional coordinates of all atoms along the y axis imagery (Table 1). Fe1 atoms are at the origin 15 16 are 0 in the brezinaite structure (Jellinek, 1957), and centre positions of the unit cell. Fe2 atoms 16 17 fractional coordinates for Fe and S atoms can be also shift slightly towards vacant sites because 17 18 measured directly from the noise-filtered Z- they share polyhedral faces with Fe1 atoms 18 19 contrast image (Fig. 6b). The measured fractional (Fig. 7). Both the presence of vacancies as well 19 20 coordinates for Fe and S atoms are also listed in as strong covalent bonding between Fe and S 20 21 Table 1 together with coordinates based on ideal cause distortion of the FeÀS6 octahedra. 21 22 hexagonal packing of S atoms. Distances between neighbouring Fe(2) atoms are 22 23 Using DFT a possible structure for the ~3.96 A˚ and 2.93 A˚ (Fig. 7) due to deviation from 23 24 monoclinic Fe3S4 at 0 K was also calculated. ideal positions of Fe atoms in vacancy-free layers. 24 25 Initial input for the DFT calculation is based on It is proposed that monoclinic Fe3S4 nano- 25 26 observed positions of atoms and vacancies as well precipitates formed through further ordering of 26 27 as measured unit-cell parameters from the FFT vacancies in pyrrhotite with a low Fe/S ratio (i.e. 27 28 pattern. The magnetic structure (spin orientation <0.875) at low temperature. Monoclinic Fe3S4 is a 28 29 for Fe atoms) of Fe3Se4 that is isomorphous with polymorph of greigite that is cubic with the 29 30 30 31 31 32 TABLE 1. Fractional coordinates of Fe and S atoms in monoclinic Fe3S4 with I2/m symmetry. 32 33 33 34 34 Atom Method used —— Fractional coordinates —— 35 35 xyz 36 36 37 Fe1 Origin 0 0 0 37 38 Fe2 Ideal 0 0 0.25 38 39 Measured 0.95(1) 0 0.26(1) 39 40 Calculated 0.9524 0 0.2589 40 41 Calculated with constrained cell* 0.9568 0 0.2616 41 42 42 S1 Ideal 0.3333 0 0.375 43 Measured 0.34(1) 0 0.36(1) 43 44 Calculated 0.3370 0 0.3599 44 45 Calculated with constrained cell* 0.3395 0 0.3618 45 46 46 47 S2 Ideal 0.3333 0 0.875 47 48 Measured 0.33(1) 0 0.89(1) 48 49 Calculated 0.3329 0 0.8916 49 Calculated with constrained cell* 0.3321 0 0.8899 50 50 51 51 52 * Note: The structure was calculated using the measured unit-cell parameters. 52 53 Measured unit-cell parameters: a = 5.94 A˚ , b = 3.43 A˚ , c = 10.70 A˚ , b = 91.0º. 53 ˚ ˚ ˚ 54 DFT calculated unit-cell parameters: a = 5.9013 A, b = 3.4267 A, c = 11.0511 A, b = 93.1º. 54

382 Z-CONTRAST IMAGING AND DFT STUDY OF NATURAL MONOCLINIC Fe3S4

1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 FIG. 7. Polyhedral model of greigite, a cubic polymorph of Fe3S4, based on DFT calculated structure (left), the 17 17 monoclinic Fe3S4 (centre) and projection of atoms along b (right). Face-sharing of neighbouring octahedra results in 18 shifts of Fe atoms towards the vacancy sites. 18 19 19 20 20 21 magnetite structure. Sulfur atoms in greigite the difference between Cr3S4 and monoclinic 21 22 (g-Fe3S4) follow cubic close packing, and Fe Fe3S4 is in the position of Cr(2) and Fe(2) and S 22 23 atoms occupy half the octahedral sites and a atoms. In the monoclinic Cr3S4 group, one Cr(1) 23 24 quarter of the tetrahedral sites (Skinner et al., atom is at the origin and centre of the unit cell, 24 25 1964). In the newly discovered monoclinic phase while the Cr(2) atoms are very near to ideal close- 25 26 (a-Fe3S4), sulfur follows hexagonal close packing packing positions (Fig. 8). However, the Fe(2) 26 27 and all Fe atoms occupy 3/4 of the octahedral atoms in monoclinic Fe3S4 clearly deviate from 27 28 sites. ideal packing positions (by ~0.3 A˚ ) along the a 28 29 The newly discovered monoclinic Fe3S4 is a direction (Fig. 8). The positions of S atoms in 29 30 subtly distorted version of brezinaite, Cr3S4. monoclinic Fe3S4 are very close to ideal packing 30 31 Compared to the monoclinic structure of Cr3S4 in monoclinic Fe3S4. However, this is not the case 31 32 (space group I2/m) determined by Jellinek (1957), for the monoclinic structure of Cr3S4 (Fig. 8). 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 50 51 51 52 52

53 FIG. 8. Projections of monoclinic Cr3S4 (left) and monoclinic Fe3S4 (right) structures along the [001] direction 53 54 showing the difference in positions of sulfur and metal atoms. 54

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1 Conclusions Microscopy. Plenum Press, New York. 1 2 Kresse, G. and Furthmu¨ller, J. (1996) Efficiency of ab- 2 3 A monoclinic form of Fe3S4 is in a topotaxial initio total energy calculations for metals and 3 4 relationship with the host pyrrhotite. The crystal semiconductors using a plane-wave basis set. 4 5 structure of the Fe3S4 nano-precipitates has Computational Materials Science, 1,15À50. 5 6 monoclinic symmetry with space group I2/m. Langenhorst, F., Harries, D. and Pollok, K. (2014) Non- 6 7 Fractional coordinates for S(1,2) and Fe(1,2) stoichiometry, defects and superstructures in sulfide 7 8 atoms within the unit cell have been determined and oxide minerals. Pp. 261À295 in: Minerals at the 8 9 from Z-contrast images and DFT methodology. Z- Nanoscale (F. Nieto and K.J.T. Livi, editors). EMU 9 Notes in Mineralogy, 14. European Mineralogical 10 contrast imaging is a powerful tool to reveal 10 Union, Eo¨tvo¨s University Press, Budapest. 11 vacancy ordering and disordering in minerals. 11 Long, G., Zhang, H., Li, D., Sabirianov, R. and Zhang, 12 This study has shown that a combination of DFT 12 Z. (2011) Magnetic anisotropy and coercivity of 13 and Z-contrast imaging methods can be used to 13 Fe3Se4 nanostructures. Applied Physics Letters, 99, 14 understand the structure of nano-minerals and 14 202103. 15 nano-precipitates. Makovicky, E. (2006) Crystal structures of sulfides and 15 16 other chalcogenides. Pp. 7À125 in: Sulfide 16 17 Acknowledgements Mineralogy and Geochemistry (D.J. Vaughan, 17 18 editor). Reviews in Mineralogy & Geochemistry, 18 19 This work is supported by NSF (EAR-095800, 61. Mineralogical Society of America and the 19 20 EAR-0810150 and DMR-0619368, MRI) and Geochemical Society, Chantilly, Virginia, USA. 20 21 NASA Astrobiology Institute (N07-5489). The Murowchick, J.B. and Barnes, H.L. (1986) Marcasite 21 22 authors thank Prof. Izabela Szlufarska for advice precipitation from hydrothermal solutions. 22 23 on DFT modelling and Dr Alex Kivit for Geochimica et Cosmochimica Acta, 50, 2615À2629. 23 24 optimizing instrument conditions. They also Naldrett, A.J., Bray, J.G., Gasparrini, E.L., Podolsky, T. 24 25 thank the Major Research Instrumentation (MRI) and Rucklidge, J.C. (1970) Cryptic variation and the 25 26 program of NSF for funding the aberration- petrology of the Sudbury Nickel Irruptive. Economic 26 27 corrected STEM at the Univeristy of Wisconsin- Geology, 65, 122À155. 27 28 Madison. Nick Levitt, Peter Williams and an Pennycook, S. (2002) Structure determination through 28 29 anonymous reviewer are thanked for helpful Z-contrast microscopy. Advances in Imaging and 29 30 comments and suggestions. Electron Physics, 123, 173À206. 30 31 Perdew, J.P., Burke, K. and Ernzerhof, M. (1996) 31 Generalized gradient approximation made simple. 32 References 32 33 Physical Review Letters, 18, 3865À3868. 33 34 Andresen, A.F. and Laar, B. (1970) The magnetic Pierce, L.P. and Buseck, P.R. (1974) Electron imaging 34 35 structure of Fe3Se4. Acta Chemica Scandinavica, 24, of pyrrhotite superstructures. Science, 186, 35 36 2435À2439. 1209À1212. 36 Posfai, M. and Buseck, P.R. (1997) Modular structures 37 Dudarev, S.L., Botton, G.A., Savrasov, S.Y., 37 in sulphides: sphalerite/wurtzite-, pyrite/marcasite-, 38 Humphreys, C.J. and Sutton, A.P. (1998) Electron- 38 and pyrrhotite-type minerals. Pp. 193À235 in: 39 energy-loss spectra and the structural stability of 39 nickel oxide: An LSDA+U study. Physical Review Modular Aspects of Minerals (S. Merlino, editor). 40 40 B, 57, 1505À1509. EMU Notes in Mineralogy, 1. European 41 41 Fleet, M.E. (1978) The pyrrhotite-marcasite transforma- Mineralogical Union, Eo¨tvo¨s University Press, 42 42 tion. The Canadian Mineralogist, 16,31À35. Budapest. 43 43 Fleet, M.E. (1982) Synthetic smythite and monoclinic Roldan, A., Santos-Carballal, D. and de Leeuw, N.H. 44 44 Fe3Se4. Physics and Chemistry of Minerals, 8, (2013) A comparative DFT study of the mechanical 45 45 241À246. and electronic properties of greigite Fe3S4 and 46 46 Hawley, J.E. (1962) The Sudbury ores: their mineralogy magnetite Fe3O4. Journal of Chemical Physics, 47 and origin. The Canadian Mineralogist, 7,1À207. 138, 204712. 47 48 Jellinek, F. (1957) The structures of the chromium Shi, F. (2013) Advanced Electron Microscopy of Novel 48 49 sulphides. Acta Crystallographica, 10, 620À628. Ferromagnetic Materials and Ferromagnet / Oxide 49 50 Keays, R.R. and Lightfoot, P.C. (2004) Formation of Interfaces in Magnetic Tunnel Junctions.PhD 50 51 NiÀCuÀPlatinum Group Element sulfide minerali- Dissertation, University of Wisconsin-Madison, 51 52 zation in the Sudbury Impact Melt Sheet. Mineralogy USA. 52 53 and Petrology, 82, 217À258. Skinner, B.J., Erd, R.C. and Grimaldi, F.S. (1964) 53 54 Kirkland, E.J. (1998) Advanced Computing in Electron Greigite, the thio-spinel of iron; a new mineral. 54

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1 American Mineralogist, 49, 543À555. American Mineralogist, 57, 1066À1080. 1 2 Taylor, L.A. and Williams, K.L. (1972) Smythite, (Fe, Xu, H., Shen, Z., Konishi, H., Fu, P. and Szlufarska, I. 2 3 Ni)9S11 À a redefinition. American Mineralogist, 57, (2014) Crystal structures of laihunite and inter- 3 4 1571À1577. mediate phases between laihunite-1M and fayalite: 4 5 Tokonami, M., Nishiguchi, K. and Morimoto, N. (1972) Z-contrast imaging and ab initio study. American 5 6 Crystal structure of a monoclinic pyrrhotite (Fe7S8). Mineralogist, 99, 881À889. 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47 48 48 49 49 50 50 51 51 52 52 53 53 54 54

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