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THE CRYSTAL STRUCTURE of ROXBYITE, Cu58s32

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THE CRYSTAL STRUCTURE of ROXBYITE, Cu58s32 423 The Canadian Mineralogist Vol. 50, pp. 423-430 (2012) DOI : 10.3749/canmin.50.2.423 THE CRYSTAL STRUCTURE OF ROXBYITE, Cu58S32 WILLIAM G. MUMME§ AND ROBERT W. GABLE CSIRO Process Science and Engineering, Box 312, Clayton South, Victoria 3169, and School of Chemistry, University of Melbourne, Parkville, Victoria, 3010, Australia VÁCLAV PETŘÍČEK Institute of Physics, ASCR v.v.i., Na Slovance 2, CZ–182 21 Praha, Czech Republic ABSTRACT Roxbyite is triclinic, space group P1, with unit-cell dimensions a 13.4051(9), b 13.4090(8), c 15.4852(3) Å, a 90.022(2), b 90.021(2), g 90.020(3)°. The crystal structure was solved using 12190 independent intensity data with I > 4s(I), out of some 43652 unique reflections measured with a single-crystal diffractometer. There are 58 different Cu atoms and 32 different S atoms in the structure. Refinement using isotropic temperature-factors yielded Rw = 0.079. The number of parameters refined was 372. Because the crystal used is twelve-fold rotationally twinned about a, 12 twin-volume parameters were invoked. The structure was determined after transformation of an initial model obtained for a unit cell four times larger, also with space group P, with a 30.9625, b 30.9727, c 13.4051 Å, a 90.007, b 89.972, g 119.994° and with a composition Cu228S128. This larger cell, which was determined by the instrumental software during data collection, actually results from complicated multiple twinning of the true cell defined above. The structure of roxbyite is based on a hexagonal-close-packed framework of sulfur atoms with the copper atoms occupying these layers, all having triangular coordination. Other layers sandwiched between the close-packed sulfur layers consist purely of double, or split, layers of Cu atoms. Some of these Cu atoms have two-fold linear coordination, but mostly they have three- and four-fold coordination to the sulfur atoms in the close-packed layers that lie above and below them. The crystal structure of roxbyite bears a strong kinship to those of low chalcocite and djurleite. Keywords: roxbyite, crystal structure, multiple twins, low chalcocite, djurleite. INTRODUCTION P21/c, with a nearly orthogonal cell, a 26.897, b 15.745, c 13.565 Å, b 90.13°. Roxbyite, Cu1.74–1.82S, was reported as a new We have determined the crystal structure of roxbyite, copper sulfide mineral occurring in the Olympic Dam and report here the results of our investigation of the deposit at Roxby Downs, South Australia, by Mumme type-locality material. et al. (1988). A presumed close structural relationship with high and low chalcocite (Evans 1971, 1979b) EXPERIMENTAL DETAILS and djurleite Cu1.96 S (Evans 1979a, 1979b) was also discussed. The single-crystal Weissenberg X-ray films The crystal of roxbyite used to collect Weissenberg of roxbyite at first indicated an orthorhombic symmetry film data in 1988 was now used for (two) data collec- with a strong superimposed hexagonal pseudosym- tions using single-crystal diffractometers situated in the metry, but the presence of some intensity variations School of Chemistry, University of Melbourne. The between what would be equivalent reflections for an first was made using the Oxford Diffraction XcaliburS orthorhombic cell suggested that it is more probably difractometer. A total 93189 reflections were collected, monoclinic (C2/m, Cm or C2), with an orthogonal cell with 12424 unique and a Rmerge value of 0.24 for the having as parameters a 53.79, b 30.90, c 13.36 Å, b 90°. presumed monoclinic symmetry. Attempts were made A further reason for initially choosing this cell is that it to solve the structure in C2/m, with close-packed layers is related metrically to the unit cell of djurleite. Djurleite of sulfur atoms containing triangularly coordinated Cu had previously been found by Evans to be monoclinic atoms alternating with split Cu-only layers, as in the § E-mail address: [email protected] 424 THE CANADIAN MINerALOGIST structure of djurleite. As discussed, the close-packing c 13.4051 Å, a 90.007, b 89.972, g119.994° (a = b was originally assumed to be normal to a ≈ 53 Å. As 30.97, c = 13.41 Å, a = b 90°, g 120°) was chosen by this approach proved to be unsatisfactory, closer atten- the instrumental software (CRYSALISPRO 2010) as the tion was given to Patterson function calculations, which preferred cell rather than a monoclinic or orthorhombic indicated that the layering is more probably normal to cell. However, for hexagonal lattice type P, the Rint was the c ≈ 13 Å axis. very high (0.221), whereas the data analysis showed A partial structure still based on that of djurleite was that, of all the possible lattice-types raised, including determined, but with only four layers of close-packed hexagonal and C-centered monoclinic, the Rint = 0.076 sulfur, with Cu in triangular coordination interleaved for triclinic symmetry was lowest. With still some with four intermediate double (or puckered) layers of doubt about the true symmetry, we decided to continue Cu in more distorted three- and four-fold coordinations. the refinement with this triclinic cell, hopefully solve However, the limited number of reflection data with the structure in P1, and later test for possible higher respect to number of atoms (with only 1322 reflections symmetry. having I > 4sI) caused the refinement to stall, even though the Fourier maps calculated by JANA (Petříček DETERMINATION OF PART TWINNING IN ROXBYITE et al. 2006) were well defined, and suggested that the modeling was progressing in the right direction. It was now possible to progress from the previous At this stage, a second set of data (Table 1) was partial refinement in monoclinicC 2/m, transposed to the collected using the newly acquired Oxford Diffrac- new triclinic cell, by using large damping constraints, tion SuperNova diffractometer. Fewer data (82771 Fourier mapping and (there being eventually 356 atoms reflections) were collected, but with greatly improved in total) least-squares refinements in selected groups of counting statistics. Notably, for this cycle of data collec- atoms. The improvements in the R value agreements tion, the pseudohexagonal cell a 30.9625, b 30.9727, were invariably incremental, although Fourier mapping continued to give quite clear images of both the sulfur– copper framework and the intermediate copper layers. TABLE 1. ROXBYITE: DATA COLLECTION AND REFINEMENT DETAILS With R ≈ 0.40, and with a complete close-packed __________________________________________________________ network of sulfur (only) and about 75% of the possible Cu atoms identified, initially six-fold rotational twin- Ideal formula Cu58 S 32 ning about c was introduced to mimic any possibility Crystal data of hexagonal symmetry. Unit-cell parameters (triclinic cell) At the very first stage, the R agreement improved a, b, c (Å) 13.4090(8), 13.4051(9), 15.4852(3) considerably. It became possible to gradually improve á, â, ã (E) 90.022(2), 90.021(2), 90.020(3) the value to an R of 0.076, with 12190 reflections with Z 2 Space group P¯1 I > 4sI. As the six twin volumes varied considerably in –3 Calculated density 5.62 g cm magnitude, it seemed that the true symmetry of roxbyite Data collection is triclinic, and therefore we eventually refined the coordinates and isotropic temperature-factors of all the Temperature (K) 293 ë (CuKá, Å) 1.5418 atoms together with twelve twin parameters required Crystal size (mm) 0.174 × 0.10 × 0.03 for P1. Fourier mapping using the CONTOUR facility Collection mode omega scan, Äù = 1.0E Count time per frame 150 seconds in JANA was critical in eventually finding all the Cu 2èmax (E) 60.98 atoms. Cell parameters used for data collection At this stage, we had now determined a crystal a, b, c (Å) 30.9625, 30.9727, 13.4051 á, â, ã (E) 90.007, 89.972, 119.994 structure for roxbyite based on a triclinic, but nearly No. unique reflections 43652 equivalent hexagonal cell, P1, with a 30.962, b 30.973, No. reflections, I > 4ó(I) 12190 Absorption correction Gaussian (indexed crystal faces) c 13.045 Å, a 90.007°, b 89.972°, g 119.994°, which –1 contained 228 Cu and 128 S independent atoms. As ì (mm ) 34.2 Tmin 0.049 such, this structure was found to have the predicted Tmax 0.432 Rmerge (observed) 0.076 relationships to the structures of low and high chalcocite Refinement and djurleite based on hexagonal close-packing of sulfur No. of parameters refined 372 layers. However, rather than the original suggestion by Mumme et al. (1988) that the close-packing in roxbyite Rwobs, I > 4ó(I) 0.079 Rwobs, all data 0.079 GOF 2.40 is normal to the a axis (53 Å), i.e., approximately eight times the axis normal to close-packing in high chal- Twinning Twelve-fold rotation around a Twin-volume fractions 0.149(5), 0.0904(16), 0.0511(14), cocite, c = 6.72 Å, four times that in low chalcocite, 0.0731(16), 0.1059(17), 0.0315(14), c = 13.494 Å, and twice that in djurleite, a = 26.879 Å, 0.1532(18), 0.0509(14), 0.0896(16), the close-packing in this model of roxbyite was actu- 0.0745(16), 0.0298(14), 0.1012(17) 3 Äómin, Äó max (e/Å ) -2.66, +3.45 ally determined to be equivalent to that found in low __________________________________________________________ THE CRYSTAL STruCTure OF ROXBYITE 425 chalcocite, having only four close-packed layers per only 142 of the weaker I > 4sI reflections are discarded cell repeat along c. from the original 12190 (~1.0%) described by the twin domains in the structure determined for Cu228S128.
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