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

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 As₂²⁻ dianions.

Related literature

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

Experimental

Crystal data

• As_1.99Co_0.61Fe_0.17Ni_0.22S_0.01

• M_r = 207.77

• Orthorhombic, P n n m

• a = 5.0669 (6) Å

• b = 5.8739 (7) Å

• c = 3.1346 (4) Å

• V = 93.29 (2) ų

• 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) 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[F² > 2σ(F²)] = 0.025

• wR(F²) = 0.061

• S = 0.91

• 254 reflections

• 13 parameters

• Δρ_max = 1.44 e Å⁻³

• Δρ_min = −1.82 e Å⁻³

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

Supporting information

S1. 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 3dσ(e_g) orbitals of M²⁺ and the π_b orbitals of As₂^2-, 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 π_b 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 [(a_Lol – a_Ram)/a_Lol: (b_Lol - b_Ram)/b_Lol: (c_Lol -c_Ram)/c_Lol] (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 As₂^2- dianions.

Related literature

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

For related literature, see: Goodenough (1967); King (2002).

S2. 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), (Co_0.61Ni_0.22Fe_0.17)_Σ=1(As_1.99S_0.01)_Σ=2, was determined with a CAMECA SX50 electron microprobe (http://rruff.info).

S3. 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.

Computing details

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

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).

Cobalt-iron-nickel diarsenide

Crystal data

As1.99Co0.61Fe0.17Ni0.22S0.01	F(000) = 168
Mr = 207.77	Dx = 7.396 Mg m^−3
Orthorhombic, Pnnm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2 2n	Cell parameters from 153 reflections
a = 5.0669 (6) Å	θ = 5.0–31.4°
b = 5.8739 (7) Å	µ = 43.75 mm^−1
c = 3.1346 (4) Å	T = 293 K
V = 93.29 (2) Å^3	Granular, black
Z = 2	0.06 × 0.05 × 0.04 mm

Data collection

Bruker APEX2 CCD area-detector diffractometer	254 independent reflections
Radiation source: fine-focus sealed tube	227 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.041
ϕ and ω scan	θmax = 36.4°, θmin = 5.3°
Absorption correction: multi-scan (TWINABS; Sheldrick, 2007)	h = −8→7
Tmin = 0.179, Tmax = 0.274	k = −8→9
1232 measured reflections	l = −5→5

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	w = 1/[σ^2(Fo^2) + (0.0428P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.061	(Δ/σ)max < 0.001
S = 0.91	Δρmax = 1.44 e Å^−3
254 reflections	Δρmin = −1.82 e Å^−3
13 parameters	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.016 (6)

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 (Ų)

	x	y	z	Uiso*/Ueq
M	0.0000	0.0000	0.0000	0.0130 (2)
As	0.18637 (9)	0.36589 (7)	0.0000	0.01033 (17)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
M	0.0090 (4)	0.0033 (4)	0.0267 (4)	0.0001 (3)	0.000	0.000
As	0.0140 (3)	0.0046 (2)	0.0124 (2)	−0.00002 (14)	0.000	0.000

Geometric parameters (Å, º)

M—As	2.3475 (5)	M—As^iii	2.3669 (4)
M—As^i	2.3475 (5)	M—As^iv	2.3669 (4)
M—As^ii	2.3669 (4)	M—As^v	2.3669 (4)
As—M—As^ii	88.016 (9)	M—As—M^vi	125.085 (13)
As^i—M—As^ii	91.984 (9)	M^vi—As—M^vii	82.931 (17)
As^ii—M—As^iv	82.931 (17)	M—As—As^viii	106.12 (3)
As^iii—M—As^iv	97.069 (17)	M^vi—As—As^viii	107.599 (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.