The Crystal Structure of Bornite Cu 5 FeS 4 : Ordered Fe and Split Cu

: The crystal structure of bornite with ideal formula Cu 5 FeS 4 from the Saishitang skarn copper deposit in Qinghai Province, along with bornite from the Yushui spouting hydrothermal copper deposit in Guangdong Province and the Bofang sandstone copper deposit in Hunan Province, has been reﬁned by single-crystal X-ray diffraction with R1 = 0.0259–0.0483 (I > 2 σ ) and 0.0338–0.1067 for 2732 to 3273 unique reﬂections. As represented by the Saishitang sample, it is orthorhombic with a Pbca space group and unit cell parameters a = 10.97016(18) Å, b = 21.8803(4) Å, c = 10.9637(2) Å, V = 2631.61(8) Å3 and Z = 16. The structure is composed of sulfur layers parallel to the (0 1 0) lattice plane with interstices occupied by metal atoms. The Fe atoms occupy two tetrahedral sites with full occupancy, but the Cu atoms are all partially distributed over 20 paired sites, split from 10 sites with a distance ranging from 0.24 Å to 0.54 Å. The Fe-S tetrahedra are not split with Fe-S lengths from 2.2609 Å to 2.3286 Å (average 2.2997 Å). The Cu-S lengths in pyramidal triangles are from 2.218 Å to 2.397 Å (average 2.288 Å), whereas the Cu-S tetrahedra are strongly distorted, with great variations in Cu-S lengths from 2.224 Å to 2.604 Å (average 2.391 Å). The orthorhombic unit cell is stacked from 16 1a-type (5.5 Å) cubes; each cube has one tetrahedrally-coordinated Fe atom, ﬁve split from 3- to 4-coordinated Cu atoms, and two vacancies, i.e., 5Cu III–IV +Fe IV +2[]+4S. The phenomenon of site-splitting of Cu atoms may provide for a more accurate structure of bornite, allowing for a better understanding of its magnetic properties and ore-formation conditions.


Introduction
Study of the crystal structure of bornite (Cu 5 FeS 4 ) first began in the 1950s and three types of structure have been reported for different stability temperatures, including the cubic Fm-3m 1a-type (a = 5.50 Å) for the high-temperature form found above 228 • C [1,2], the cubic Fm-3m 2a-type for the intermediate form of 170-235 • C [3][4][5] and the orthorhombic Pbca 2a-4a-2a-type for the low-temperature form below 200 • C [6,7]. In addition, the cubic Fm-3m 4a-type [8] and the rhombohedral R3m type [2] were also proposed for the intermediate form. In the early reported structure [7], the metal atoms are mixed over tetrahedral or triangular sites, which seems to disagree with the highly stoichiometric composition of bornite (Cu 5 FeS 4 ) and the common knowledge that isomorphic substitution between Cu and Fe is quite limited in nature. Martinelli et al. first identified tetrahedral sites that were fully occupied by Fe in Pbca bornite at 275 • K and observed a structural transition from Pbca to Pca2 1 at 65 • K using synchrotron powder X-ray diffraction data [9]. However, a study of single crystal X-ray diffraction is still needed to clarify the locations of Fe and Cu atoms in the structure to satisfy the stoichiometric Cu 5 FeS 4 in natural bornite. Inspired by this, we collected bornite samples from different localities representative of the skarn, spouting hydrothermal and sandstone copper deposits in China, and carried out structural determination experiments using single-crystal X-ray diffraction.

Samples and Experimental Methods
The bornite samples were collected from the Saishitang skarn copper deposit in Qinghai Province, the Yushui spouting hydrothermal porphyry copper deposit in Guangdong Province and the Bofang sandstone copper deposit in Hunan Province. The polished sections of the samples were observed using a Leica DM2500p microscope manufactured by Shimadzu company of Japan ( Figure 1). In Saishitang, bornite is associated with stannoidite, chalcopyrite and cobaltite in a matrix of andradite. In Yushui, betekhtinite, chacocite and stromeyerite are disseminated in massive bornite. In Bofang, bornite, along with tetrahedrite, is disseminated in a matrix of chalcopyrite. The compositions were analyzed with a Shimadzu-1720 electron probe microanalyzer (EPMA) at accelerating voltage 15 kV, beam current 10 nA and beam size 1µm. Pure materials of Cu, Ag and FeS 2 were used as standards for the quantification of Cu (Kα), Ag (Lα), Fe (Kα) and S (Kα) using ZAF correction. Five points were analyzed for each of the three samples and the average weight percent and empirical formula are shown in Table 1, which indicates a stoichiometric formula Cu 5 FeS 4 .

Samples and Experimental Methods
The bornite samples were collected from the Saishitang skarn copper deposit in Qinghai Province, the Yushui spouting hydrothermal porphyry copper deposit in Guangdong Province and the Bofang sandstone copper deposit in Hunan Province. The polished sections of the samples were observed using a Leica DM2500p microscope manufactured by Shimadzu company of Japan ( Figure 1). In Saishitang, bornite is associated with stannoidite, chalcopyrite and cobaltite in a matrix of andradite. In Yushui, betekhtinite, chacocite and stromeyerite are disseminated in massive bornite. In Bofang, bornite, along with tetrahedrite, is disseminated in a matrix of chalcopyrite. The compositions were analyzed with a Shimadzu-1720 electron probe microanalyzer (EPMA) at accelerating voltage 15 kV, beam current 10 nA and beam size 1μm. Pure materials of Cu, Ag and FeS2 were used as standards for the quantification of Cu (Kα), Ag (Lα), Fe (Kα) and S (Kα) using ZAF correction. Five points were analyzed for each of the three samples and the average weight percent and empirical formula are shown in Table 1, which indicates a stoichiometric formula Cu5FeS4.  The single-crystal diffraction data were collected using a Rigaku XtaLAB Synergy-DW diffractometer with microfocus sealed Mo and Cu anode tubes at 50 kV and 1 mA. According to the crystal size and X-ray tube intensity, the exposure time per frame was 20, 10 and 80 s, respectively, for the samples from Saishitang, Yushui and Bofang. The experimental data were treated with CrysAlisPro and all reflections were indexed on the basis of an orthorhombic (pseudotetragonal) unit cell ( Table 2). The intensity data were corrected for X-ray absorption using the Rigaku program ABSPACK. The systematic absence of reflections is suggestive of the space group Pbca. The crystal structure was then solved for the space group Pbca with SHELXT [10] and refined with SHELXL [11], both  The single-crystal diffraction data were collected using a Rigaku XtaLAB Synergy-DW diffractometer with microfocus sealed Mo and Cu anode tubes at 50 kV and 1 mA. According to the crystal size and X-ray tube intensity, the exposure time per frame was 20, 10 and 80 s, respectively, for the samples from Saishitang, Yushui and Bofang. The experimental data were treated with CrysAlisPro and all reflections were indexed on the basis of an orthorhombic (pseudotetragonal) unit cell ( Table 2). The intensity data were corrected for X-ray absorption using the Rigaku program ABSPACK. The systematic absence of reflections is suggestive of the space group Pbca. The crystal structure was then solved for the space group Pbca with SHELXT [10] and refined with SHELXL [11], both included in the freeware Olex2 [12]. The structure model and site labels of Koto and Morimoto were adopted, in which the split M sites are distinguished by a and b (Table 2) [7]. The positions of atoms and anisotropic displacement parameters were refined with full occupancies for S and Fe, and free occupancies for Cu at split sites ( Table 3). The structures were illustrated with the freeware VESTA [13].  Table 3. Fractional atomic coordinates and displacement parameters (Å 2 ) of atoms in bornite from Saishitang. Cu

Results and Discussion
The samples of bornite from the three localities all show the same structure, although the refinement R1 varies from 0.0259 in the Saishitang samples to 0.0483 in those from Bofang (for reflections with I > 2σ(I)) due to different crystal qualities ( Table 2). The structure is comparable to those of Koto and Morimoto and Martinelli et al., but differs from the former due to the separate occupation of Fe and Cu, and differs from the latter in terms of the site-splitting of Cu atoms [7,9]. The coordinates and displacement parameters of atoms in bornite, represented by those of the Saishitang samples (.cif files of the other two localities are contained within the supplementary materials and are available as needed) are listed in Table 3, with selected bond distances and angles shown in Table 4. The results reveal that the two tetrahedral sites (M4 and M5) are uniquely occupied by Fe atoms without splitting. Cu atoms, however, are partially distributed over 20 paired sites, which are split from 10 M sites. The site-splitting is indispensable as it rapidly improves R1 from 0.0764, and the residual peaks from 3.79/−3.80 e − Å −3 before splitting to 0.0259 and 1.15/−0.71 e − Å −3 after splitting. To confirm the lack of Fe-Cu mixed occupation, a refinement of free occupancy for all cation sites was also tried and the results indicated that the occupancies of Fe at M4 and M5 range from 0.998 to 1.006, and that the sum occupancies of Cu at split M sites vary between 0.975 and 0.997 for all three samples, in almost perfect agreement with stoichiometric Cu 5 FeS 4 with negligible Fe-Cu mixing. The crystal structure of bornite is composed of layers of S atoms parallel to (010) with interstices filled by metal atoms (Figure 2), as described for cubic 1a-, 2a-type bornite and orthorhombic 2a4a2a-type bornite [1,2,4,7]. The interlayer distance of the S-layer in bornite is 2.735 Å and the 12 closest S-S lengths range from 3.734 Å to 3.989 Å (average 3.841 Å). This is similar to the S-S lengths of 3.878-3.889 Å reported for the cubic 1a-type, 2a-type bornite and the orthorhombic bornite, but bigger than the layer interval (2.603 Å) and the S-S length (3.719 Å) in chalcopyrite [5].
In the literature, the structure of orthorhombic bornite was stacked from fully filled In the literature, the structure of orthorhombic bornite was stacked from fully filled antifluorite 1a cubes and half-filled sphalerite 1a cubes [4,14]. Following this, we numbered each position of the 8 interstices in the 1a-type (5.5 Å) cube from 1-8 consisting of four face-centered S atoms. The cube was labeled 1, 1 to 8, 8 according to the position of the Fe atom and the setting of the two possible vacancies for the same position of Fe in the cube ( Figure 3c). As such, the 2a-4a-2a orthorhombic unit cell of bornite is a building block of 16 cubes of the 1a type each with a number 1, 1 to 8, 8 (Figure 3a-c). In each cube, five Cu atoms are located in 10 paired sites split from three tetrahedral and two pyramidal triangle sites (Figures 2 and 3). Thus, the cube chemistry may be written as 5Cu III-IV +Fe IV +2[]+4S in accordance with the stoichiometric formula Cu 5 FeS 4 . Similarly, the crystal structure of chalcopyrite is a 1a-1a-2a stacking supercell of the cubes labelled as 2-7 and 4-5 according to the position number of Fe in the cube, which contains two Fe atoms and two Cu atoms in the tetrahedral interstices, together with four fixed vacancies (i.e., 2Cu IV + 2Fe IV + 4[] + 4S), as shown in Figure 3d- 3+ and Cu + as shown by similar Mössbauer data (I.S. 0.12-0.53, Q.S., 0-0.50) of bornite and chalcopyrite [15].  The bond valence sums for Fe atoms at M4 and M5 range from 2.48 to 2.49 whereas the bond valence sums for Cu atoms at various sites vary between 0.92 and 1.02. These are comparable to those of chalcopyrite (2.74 for Fe, 1.19 for Cu) and are indicative of the valence states of Fe 3+ and Cu + as shown by similar Mӧssbauer data (I.S. 0.12-0.53, Q.S., 0-0.50) of bornite and chalcopyrite [15].

Conclusions and Implications
From this single-crystal X-ray diffraction study of samples from a number of different deposits we confirmed the orthorhombic structure of bornite, the separate occupation of Fe at two tetrahedral sites, and the partial occupation of Cu over 20 paired sites split from 10 fully occupied sites with negligible Fe-Cu mixing. We also proposed a model of a building block of 16 cubes of the 1a type with exact indication of the position of Fe and Cu atoms in the cube as well as any vacancies in accordance with the stoichiometric formula Cu5FeS4.
Bornite is an important metal sulfide in various copper deposits and has been described and studied for over 100 years [16][17][18]. The phenomenon of site-splitting of Cu atoms in orthorhombic Pbca bornite may also exist in bornite with different structure types alongside other sulfides. This may provide evidence of a more accurate structure of

Conclusions and Implications
From this single-crystal X-ray diffraction study of samples from a number of different deposits we confirmed the orthorhombic structure of bornite, the separate occupation of Fe at two tetrahedral sites, and the partial occupation of Cu over 20 paired sites split from 10 fully occupied sites with negligible Fe-Cu mixing. We also proposed a model of a building block of 16 cubes of the 1a type with exact indication of the position of Fe and Cu atoms in the cube as well as any vacancies in accordance with the stoichiometric formula Cu 5 FeS 4 .
Bornite is an important metal sulfide in various copper deposits and has been described and studied for over 100 years [16][17][18]. The phenomenon of site-splitting of Cu atoms in orthorhombic Pbca bornite may also exist in bornite with different structure types alongside other sulfides. This may provide evidence of a more accurate structure of bornite and could lead to a better understanding of its magnetic properties and ore-formation conditions [19][20][21].

Data Availability Statement:
The data that support the research of this study are available from the corresponding author upon reasonable request.