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Acta Crystallogr Sect E Struct Rep Online. 2008 September 1; 64(Pt 9): i62.
Published online 2008 August 23. doi:  10.1107/S1600536808026688
PMCID: PMC2960506

Safflorite, (Co,Ni,Fe)As2, isomorphous with marcasite

Abstract

Safflorite, a naturally occurring cobalt-nickel-iron diarsenide (Co,Ni,Fe)As2, possesses the marcasite-type structure, with cations (M = Co + Ni + Fe) at site symmetry 2/m and As anions at m. The MAs6 octa­hedra share two edges, forming chains parallel to c. The chemical formula for safflorite should be expressed as (Co,Ni,Fe)As2, rather than the end-member format CoAs2, as its structure stabilization requires the simultaneous inter­action of the electronic states of Co, Ni, and Fe with As2 2− dianions.

Related literature

For related literature, see: Anawar et al. (2003 [triangle]); Carlon & Bleeker (1988 [triangle]); Darmon & Wintenberger (1966 [triangle]); Ennaciri et al. (1995 [triangle]); Goodenough (1967 [triangle]); Grorud (1997 [triangle]); Hem et al. (2001 [triangle]); Holmes (1947 [triangle]); King (2002 [triangle]); Kjekshus (1971 [triangle]); Kjekshus et al. (1974 [triangle], 1979 [triangle]); Lutz et al. (1987 [triangle]); Makovicky (2006 [triangle]); O’Day (2006 [triangle]); Ondrus et al. (2001 [triangle]); Palenik et al. (2004 [triangle]); Petruk et al. (1971 [triangle]); Radcliffe & Berry (1968 [triangle], 1971 [triangle]); Reich et al. (2005 [triangle]); Robinson et al. (1971 [triangle]); Swanson et al. (1966 [triangle]); Tossell (1984 [triangle]); Tossell et al. (1981 [triangle]); Vaughan & Rosso (2006 [triangle]); Wagner & Lorenz (2002 [triangle]).

Experimental

Crystal data

  • As1.99Co0.61Fe0.17Ni0.22S0.01
  • M r = 207.77
  • Orthorhombic, An external file that holds a picture, illustration, etc.
Object name is e-64-00i62-efi1.jpg
  • a = 5.0669 (6) Å
  • b = 5.8739 (7) Å
  • c = 3.1346 (4) Å
  • V = 93.29 (2) Å3
  • Z = 2
  • Mo Kα radiation
  • μ = 43.75 mm−1
  • T = 293 (2) K
  • 0.06 × 0.05 × 0.04 mm

Data collection

  • Bruker APEX2 CCD area-detector diffractometer
  • Absorption correction: multi-scan (TWINABS; Sheldrick, 2007 [triangle]) T min = 0.179, T max = 0.274 (expected range = 0.114–0.174)
  • 1232 measured reflections
  • 254 independent reflections
  • 227 reflections with I > 2σ(I)
  • R int = 0.041

Refinement

  • R[F 2 > 2σ(F 2)] = 0.025
  • wR(F 2) = 0.061
  • S = 0.91
  • 254 reflections
  • 13 parameters
  • Δρmax = 1.44 e Å−3
  • Δρmin = −1.82 e Å−3

Data collection: APEX2 (Bruker, 2003 [triangle]); cell refinement: SAINT (Bruker, 2005 [triangle]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003 [triangle]); software used to prepare material for publication: SHELXTL (Sheldrick, 2008 [triangle]).

Supplementary Material

Crystal structure: contains datablocks I, New_Global_Publ_Block. DOI: 10.1107/S1600536808026688/mg2054sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808026688/mg2054Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors gratefully acknowledge the support of this study from the RRUFF project and NSF (EAR-0609906) for the study of bonding systematics in sulfide minerals.

supplementary crystallographic information

Comment

Minerals in the FeAs2—NiAs2—CoAs2 system include löllingite FeAs2, rammelsbergite NiAs2, pararammelsbergite NiAs2, clinosafflorite CoAs2, and safflorite CoAs2. These diarsenide minerals, together with Fe—Ni—Co disulfides and sulfarsenides, are commonly found in complex Co—Ni—As ore deposits, such as Håkansboda, Sweden (Carlon & Bleeker, 1988), the Cobalt District, Ontario (Petruk et al., 1971), Bou-Azzer, Morocco (Ennaciri et al., 1995), Modum, Norway (Grorud, 1997), and Spessart, Germany (Wagner & Lorenz, 2002). When precipitated from hydrothermal solutions, these minerals can incorporate considerable amounts of trace metals, especially so-called ''invisible'' gold (e.g., Palenik et al., 2004; Reich et al., 2005). Under oxidizing conditions, however, they can release significant amounts of arsenic into natural water and soils, in some cases producing serious arsenic poisoning and contamination (King, 2002; Anawar et al., 2003; O'Day, 2006). Therefore, the crystal structures and bonding models of Fe—Ni—Co disulfides, diarsenides, and sulfarsenides have been a subject of extensive experimental and theoretical studies (Vaughan & Rosso, 2006; Makovicky, 2006, and references therein)

The crystal structures of all minerals, except safflorite, in the FeAs2—NiAs2—CoAs2 system have been determined. Topologically, löllingite FeAs2 (Kjekshus et al., 1979; Lutz et al., 1987; Ondrus et al., 2001) and rammelsbergite NiAs2 (Kjekshus et al., 1974, 1979) possess the marcasite (FeS2)-type structure with space group Pnnm, whereas clinosafflorite CoAs2 (Darmon & Wintenberger, 1966; Kjekshus, 1971) is isostructural with the modified marcasite-type structure of arsenopyrite (FeAsS) with space group P21/c (Hem et al., 2001; Makovicky, 2006). From the unit-cell dimensions measured from X-ray diffraction, safflorite was assumed to be isotypic with marcasite (Holmes, 1947; Radcliffe & Berry, 1968, 1971). Chemical analyses of various natural and synthetic samples reveal that Pnnm safflorite always contains some amounts of Fe and Ni, whereas materials with 80–100% (mole) CoAs2 crystallize in monoclinic P21/c symmetry (Holmes, 1947; Swanson et al., 1966; Radcliffe & Berry, 1971). This study presents the first structure determination of safflorite based on single-crystal X-ray diffraction data.

Safflorite is isomorphous with marcasite. Each cation (M = Co, Ni, and Fe) at site symmetry 2/m is octahedrally coordinated by six anions (As) at site symmetry m and each anion is tetrahedrally bonded to another anion (forming As—As dianion units) plus three M cations. The MAs6 octahedra share two edges, forming chains parallel to c, and two vertices with adjacent chains (Fig. 1). The average M—As bond distance (2.360 Å) is identical to that in clinosafflorite (Kjekshus, 1971), but slightly shorter than that in löllingite (2.379 Å) (Kjekshus et al., 1979; Lutz et al., 1987) or rammelsbergite (2.378 Å) (Kjekshus et al., 1979). Notably, as the d-orbital electrons in M cations increase from Fe (d = 6) in löllingite to Co (d = 7) in safflorite, and Ni (d = 8) in rammelsbergite, the M—M separation along the chain direction increases significantly from 2.882 to 3.134, and 3.545 Å, respectively, while the As—As edge length shared by the two M octahedra concomitantly decreases from 3.808 to 3.547, and 3.219 Å. The octahedral distortion, measured by the octahedral angle variance (OAV) and quadratic elongation (OQE) (Robinson et al., 1971), decreases. The OAV and OQE values are 92.87 and 1.0265 for FeAs6, 21.04 and 1.0058 for CoAs6, and 16.22 and 1.0049 for NiAs6.

The variation of the M—M separation with the number of d-orbital electrons in marcasite-type disulfides, diarsenides, and sulfarsenides has been a matter of discussion (see Vaughan & Rosso, 2006 for a thorough review). Theoretical calculations based on molecular orbital and band models predict that, due to the interaction between the 3(eg) orbitals of M2+ and the πb orbitals of As22-, the M—As—M angle subtending the M—M separation across the shared octahedral edge should be substantially smaller for FeAs2 than for CoAs2 and NiAs2, resulting in the so-called ''compressed marcasite-type'' structure (Tossell et al., 1981; Tossell, 1984). Indeed, this angle is 74° in FeAs2 löllingite (Lutz et al., 1987), but 83° in (Co,Ni,Fe)As2 safflorite and 96° in rammelsbergite (Kjekshus et al., 1979). It is intriguing to note that the end-member CoAs2 has been found to only crystallize in the arsenopyrite-type structure (P21/c) (Holmes, 1947; Swanson et al., 1966; Radcliffe & Berry, 1971), rather than the marcasite-type structure (Pnnm). This observation may be explained by the existence of an unpaired electron occupying one of the πb orbitals, which splits into a lower-energy filled band and a higher-energy empty band (Goodenough, 1967), thus resulting in the symmetry reduction from Pnnm to P21/c. In other words, the presence of some Ni/Fe in place of Co appears to be an essential requirement for the CoAs2 system to crystallize in the Pnnm symmetry. The pure system will otherwise be stabilized energetically in the clinosafflorite structure.

Another outstanding feature of the safflorite structure is the prominent anisotropic displacement ellipsoid of the M cation, the U11:U22:U33 ratio being approximately 3:1:9, with the ellipsoid axial directions roughly parallel to the unit cell axes. This ratio can be compared to the differences of three unit-cell dimensions between FeAs2 löllingite and NiAs2 rammelsbergite [(aLol aRam)/aLol: (bLol - bRam)/bLol: (cLol -cRam)/cLol] (Kjekshus et al., 1974, 1979; Lutz et al., 1987), which is about 3:1:8. Accordingly, the marked anisotropy of the displacement parameters of the M cation in safflorite is interpreted as a consequence of positional disorder with Fe and Ni occupying apparent different positions, which in turn results from the different interactions of their d-electrons with the As22- dianions.

Experimental

The safflorite specimen used in this study is from Timiskaming County, Ontario, Canada, and is in the collection of the RRUFF project (deposition No. R070611; http://rruff.info), donated by James Shigley. The average chemical composition (15 point analyses), (Co0.61Ni0.22Fe0.17)Σ=1(As1.99S0.01)Σ=2, was determined with a CAMECA SX50 electron microprobe (http://rruff.info).

Refinement

Due to the similar X-ray scattering powers for Co, Ni, and Fe, all cations were assumed to be Co and their site occupancies were not determined during the refinement. All crystals examined were twinned, with {011} as twin plane. The structure refinements were performed based on X-ray diffraction data collected from a twinned crystal, which were processed with TWINABS (Sheldrick, 2007). The ratio of two twin components is 0.73:0.27. The highest residual peak in the difference Fourier maps was located at (0.133, 0.370, 0.256), 0.85 Å from atom As, and the deepest hole at (0.133, 0.473, 0), 0.69 Å from As.

Figures

Fig. 1.
Crystal structure of safflorite, with displacement ellipsoids drawn at the 99.9% probabiliy level. The M (=Co+Ni+Fe) cations (yellow spheres) are situated in octahedra coordinated by six As atoms (pink spheres).

Crystal data

As1.99Co0.61Fe0.17Ni0.22S0.01F000 = 168
Mr = 207.77Dx = 7.396 Mg m3
Orthorhombic, PnnmMo Kα radiation λ = 0.71073 Å
Hall symbol: -P 2 2nCell parameters from 153 reflections
a = 5.0669 (6) Åθ = 5.0–31.4º
b = 5.8739 (7) ŵ = 43.75 mm1
c = 3.1346 (4) ÅT = 293 (2) K
V = 93.29 (2) Å3Granular, black
Z = 20.06 × 0.05 × 0.04 mm

Data collection

Bruker APEX2 CCD area-detector diffractometer254 independent reflections
Radiation source: fine-focus sealed tube227 reflections with I > 2σ(I)
Monochromator: graphiteRint = 0.041
T = 293(2) Kθmax = 36.4º
[var phi] and ω scanθmin = 5.3º
Absorption correction: multi-scan(TWINABS; Sheldrick, 2007)h = −8→7
Tmin = 0.179, Tmax = 0.274k = −8→9
1232 measured reflectionsl = −5→5

Refinement

Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: full  w = 1/[σ2(Fo2) + (0.0428P)2] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.025(Δ/σ)max < 0.001
wR(F2) = 0.061Δρmax = 1.44 e Å3
S = 0.91Δρmin = −1.82 e Å3
254 reflectionsExtinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
13 parametersExtinction coefficient: 0.016 (6)
Primary atom site location: structure-invariant direct methods

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

xyzUiso*/Ueq
M0.00000.00000.00000.0130 (2)
As0.18637 (9)0.36589 (7)0.00000.01033 (17)

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
M0.0090 (4)0.0033 (4)0.0267 (4)0.0001 (3)0.0000.000
As0.0140 (3)0.0046 (2)0.0124 (2)−0.00002 (14)0.0000.000

Geometric parameters (Å, °)

M—As2.3475 (5)M—Asiii2.3669 (4)
M—Asi2.3475 (5)M—Asiv2.3669 (4)
M—Asii2.3669 (4)M—Asv2.3669 (4)
As—M—Asii88.016 (9)M—As—Mvi125.085 (13)
Asi—M—Asii91.984 (9)Mvi—As—Mvii82.931 (17)
Asii—M—Asiv82.931 (17)M—As—Asviii106.12 (3)
Asiii—M—Asiv97.069 (17)Mvi—As—Asviii107.599 (2)

Symmetry codes: (i) −x, −y, −z; (ii) x−1/2, −y+1/2, z−1/2; (iii) −x+1/2, y−1/2, −z+1/2; (iv) x−1/2, −y+1/2, z+1/2; (v) −x+1/2, y−1/2, −z−1/2; (vi) −x+1/2, y+1/2, −z+1/2; (vii) −x+1/2, y+1/2, −z−1/2; (viii) −x, −y+1, −z.

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: MG2054).

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Articles from Acta Crystallographica Section E: Structure Reports Online are provided here courtesy of International Union of Crystallography