American Mineralogist, Volume 94, pages 1405–1410, 2009

The crystal structure of a naturally occurring 5C pyrrhotite from Sudbury, its chemistry,and vacancy distribution
Johan P.R. de VillieRs,1,* daVid C. liles,2 and Megan BeCkeR1
Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa
2
Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa

1

aBstRaCt
The crystal structure of a naturally occurring pyrrhotite from the Copper Cliff North Mine, Sudbury,
Canada, has been determined in the space group Cmce (formerly Cmca), and the positions and occupancy of the Fe and S atoms have been refined, together with their anisotropic atomic displacement
parameters, to a conventional R value of 0.072. The summed occupancies of the Fe and Ni atoms[8.908 (Fe + Ni) per 10 S] are compared with the chemical composition, as determined by microprobe
analysis, the latter giving a formula of Fe8.892Ni0.115S10 or 9.007 (Fe + Ni) per 10 S.
The structure has cell dimensions of a = 6.893(3) Å, b = 11.939(3) Å, and c = 28.63(1) Å. The S
atoms are hexagonally close packed and adjacent Fe-S octahedra share faces that are perpendicular
to the c direction. The Fe atoms in adjacent face-sharing octahedra also have the closest Fe-Fe interatomic distances. Some Fe atoms have partial occupancies, and together with the fully occupied sites,
they add up to 71.26 (Fe + Ni) atoms per 80 S atoms in the unit cell, as compared with 72 atoms per
80 S atoms for Fe9S10.
The distribution of Fe in the partially occupied layers is different from that previously postulated
for Fe9S10, with vacant sites present in two of the 10 layers. The partially occupied Fe sites are present in all the remaining cation layers, and there are therefore no cation layers containing completely
filled atomic sites. Space group symmetry constrains the partially occupied cation sites to project on
top of each other along the c axis, similar to the distribution of partially occupied sites in Fe11S12 (6C
pyrrhotite).
Keywords: Pyrrhotite-5C, crystal structure, vacancy distribution, non-stoichiometry

intRoduCtion
Pyrrhotite group minerals are important naturally occurring
sulfides, and they are described as non-stoichiometric iron monosulfides having a general formula Fe1–xS, with x ranging from 0
to 0.125. They are derivatives of the hexagonally close-packed
NiAs structure, and the group consists of troilite (FeS) and pyrrhotites with continuously variable compositions that include
Fe7S8, Fe9S10, Fe10S11, and Fe11S12. Wang and Salveson (2005) givea recent review of the structures and properties of pyrrhotite. The
structures of the pyrrhotites are complex and some are incommensurate, so that conventional structure analysis tools are notadequate to describe them. The modulated structure of Fe10S11
has been described by Yamamoto and Nakazawa (1982) using
a four-dimensional description of the structure, with the fourth
dimension describing the compositional and displacement modulations along the c* direction. Izaola et al. (2007) revised thisformulation, and a common model based on a centrosymmetricsuperspace group was proposed for the pyrrhotite family.
Morimoto et al. (1975) classified the pyrrhotites as either NAor NC types depending on the superstructures that develop as aresult of the ordering of vacancies in the structures. In spite ofnumerous studies of pyrrhotites as a series of non-stoichiometriccompounds, only two naturally occurring commensurate crystal* E-mail: johan.devilliers@up.ac.za
0003-004X/09/0010–1405$05.00/DOI: 10.2138/am.2009.3081

structures could be found in the literature. These are the structuresof troilite, FeS (Evans 1970) and magnetic pyrrhotite, Fe7S8(Tokonami et al. 1972). So, if Rietveld refinement of natural
pyrrhotites or their quantification is envisaged, their crystal
structures, with the exception of Fe7S8, are not available, andrefinement and quantification is thus not possible.
The so-called hexagonal pyrrhotite or non-magnetic pyrrhotite has been described from its Sudbury locality in Canada
by Desborough and Carpenter (1965). Its symmetry and celldimensions have been described as trigonal with cell dimensionsa = 6.893 Å and c = 28.635 Å. Morimoto et al. (1975), in theirreview of the pyrrhotites, mention that the 5C structure is actuallyorthorhombic with space group (i.e., aspect symbol) C*ca, withcell dimensions a = 6.885 Å, b = 11.944 Å, and c = 28.676 Å.
Due to the renewed interest in the flotation properties of pyrrhotite, which is an important accessory sulfide in the productionof nickel and platinum-group elements, its characterization andquantification are of importance. Traditionally, the pyrrhotites
have been classified as either magnetic monoclinic pyrrhotite,or non-magnetic hexagonal pyrrhotite, and their distinction andquantification are required when interpreting their flotation
behavior. Graham (1969) devised a method to determine therelative amounts of monoclinic and hexagonal pyrrhotite basedon the relative intensities of the individual components of the
split strongest powder X-ray diffraction peak. The developmentof more precise quantification of coexisting pyrrhotites using

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DE VIllIERS ET Al.: CRYSTAl STRUCTURE OF A 5C PYRRhOTITE

whole pattern refinement methods is dependent on the existenceof refined crystal structure data. This is presented here for the
non-magnetic 5C member of this important mineral group.

exPeRiMental Methods
Massive sulfide samples derived from the 100 ore body at Copper Cliff Northmine in Sudbury, Canada, were provided by Vale INCO and pyrrhotite from themwas selected for investigation. Numerous pyrrhotite grains were subsequently
compositionally analyzed (Fig. 1) on a Jeol JXA 8100 Superprobe housed at
the University of Cape Town, using an accelerating voltage of 25 kV and probe
current of 20 nA. The compositional range of Copper Cliff North pyrrhotite as
shown in Figure 1 illustrates that most compositions plot on the sulfur-rich sideof the theoretical Fe9S10 composition, with the maximum at 8.9 to 8.95 metal (Fe+ Ni + Cu + Co) atoms per 10 S atoms. Also apparent is the continuous range incomposition of these pyrrhotite specimens.
Numerous single crystals were examined, and only one suitable for singlecrystal analysis was identified. After single-crystal data collection, the crystal was
analyzed with a Cameca SX100 electron microprobe at the University of Pretoria,
run at 20 kV accelerating voltage and 20 nA probe current. The average composition,
based on 6 analyses and normalized to 10 S atoms, is given in Table 1. The average
analysis for the crystal is 52.61 atom% S, 46.78 at% Fe, and 0.61 at% Ni. Initially,
the structure analysis was attempted with a hexagonal cell and space group P63/m,
but this was not successful. The unit-cell and diffraction data were then transformed
to an orthorhombic setting based on the suggestion by Morimoto et al. (1975) thatthe symmetry is orthorhombic with space group C*ca. Data were collected on a
Bruker (Siemens) P4 diffractometer equipped with a Bruker SMART 1K CCD
detector using graphite crystal monochromated MoKα radiation by means of a
combination of phi and omega scans. Data reduction was performed using SAINT+(Bruker 2001), and the intensities were corrected for absorption by multiple scansof symmetry equivalent reflections using SADABS (Bruker 2001). The intensitiesof 1264 unique reflections (sorted from 6041 reflections) were used in the refinements and the internal discrepancy factor of R(int) = 0.032 was obtained, which
was considered to be acceptable. The crystal data are given in Table 1.
Structure determination was achieved using the ShElXD program (Sheldrick1997) and refinement proceeded with 72 Fe atoms and 80 S atoms. Initially, becauseof the strong correlation between occupancy and atomic displacement parameters(ADP), an overall ADP for the Fe atoms was refined to establish the individual
occupancy of the Fe atoms. Additional electron density was found at the position(0,0,½) and this was included in the refinement. The Fe sites that refined to occupancies close to 1.0 were then fixed at unity, and the individual isotropic ADPs
were then allowed to vary, together with the occupancy of 3 Fe atoms. Neutral

scattering factors were used for all atoms. A small amount of Ni was allocated to
the Fe1 site to account for its presence in the crystal. lastly, the ADPs were refined
anisotropically, together with the occupancies of the 3 Fe atoms. The refinement
converged with a discrepancy factor R1 = 0.069 for 915 observed reflections andR1 = 0.090 for all 1264 measured reflections and 114 parameters. This included
an extinction correction. After refinement, the goodness of fit parameter was S =1.331. The largest residual electron density amounted to 1.13 e/Å. Because of an
unreasonably large ADP for the Fe atom at (0,0,½), it was then removed from therefinement. R1 increased marginally to 0.0716 for 915 observed reflections and0.0937 for 1264 measured reflections and 109 parameters. The goodness of fit
parameter improved to 1.225. Residual electron density at position (0,0,½) afterrefinement was slightly higher at 1.53 e/Å3, but is still considered acceptable withthe second highest density at 1.22 e/Å3. The ADPs of all atoms are now much
more realistic and the very high ADP of the atom previously placed at (0,0,½) isof course eliminated. No further improvement in the refinement was achieved andthe larger than normal R factor is thought to be a result of possible planar defects
or stacking faults occurring in some layers in the structure. This was describedby Posfai et al. (2000) using transmission electron microscopy. The composition,
calculated from the refinement of the structure, is Fe8.79Ni0.118S10.
The atomic positional and anisotropic displacement parameters are given in
Tables 2 and 3, respectively, with the standard errors in parentheses. Table 4 gives
the bond lengths, octahedral angular variance (OAV), and polyhedral quadratic

Table 1.

VolumeZ, Calculated density
Absorption coefficientF(000)
Crystal sizeθ range for data collection
Limiting indices
Reflections collected/uniqueCompleteness to θ = 25.00
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2θ(I)]R indices (all data)
Extinction coefficient
Largest diff. peak and hole

Table 2.

FiguRe 1. Distribution of 5C pyrrhotite compositions from the
Copper Cliff North mine. The maximum in the distribution is in therange of 8.9 to 8.95 metal (Fe + Ni + Cu + Co) per 10 S.

Crystal data and structure refinement for 5C pyrrhotite

Identification code
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system, space group
Unit-cell dimensions

pyrrhotite-5CFe8.79Ni0.118S10
6588.86
293(2) K
0.71073 Å
Orthorhombic, Cmce (formerly Cmca)
a = 6.893(3) Å
α = 90°b = 11.939(3) Å
β = 90°c = 28.635(12) Å
γ = 90°2356.4(15) Å3
38, 4.643 Mg/m
12.516 mm–13154
0.14 × 0.06 × 0.03 mm32.85–26.34°–8 ≤ h ≤ 7, –8 ≤ k ≤ 14, –33 ≤ l ≤ 29
6041/1264 [R(int) = 0.0322]
99.2%
Full-matrix least-squares on F2
1264 / 0 / 109
1.225
R1 = 0.0716, wR2 = 0.1513
R1 = 0.0937, wR2 = 0.1627
0.00003(2)1.533 and –2.125 e·Å–3

Positional parameters, occupancies, and equivalent isotropic atomic displacement parameters of the atoms inpyrrhotite-5C

Atom
x
y
z
Occ
Ueq
Fe1
0.50000.0
0.50000.76
0.019(1)0.019(1)Ni1
0.24
0.25000.50001.0
0.019(1)Fe20.2500–0.0017(2)0.6978(1)1.00.018(1)Fe3
0.0
0.0140(2)0.6007(1)1.00.016(1)Fe40.50000.4855(2)0.7008(1)1.00.022(1)Fe5
0.0–0.0012(2)0.5947(1)0.857(10)0.018(1)Fe6
0.0
0.2395(1)0.6015(1)1.00.017(1)Fe7
0.2732(2)0.2569(2)0.7015(1)0.775(7)0.018(1)Fe80.2311(3)0.1683(3)0.4466(1)1.00.012(1)S10.50000.1650(3)0.6483(1)1.00.008(1)S2
0.0
0.3326(3)0.7531(1)1.00.008(1)S3
0.0
0.0806(3)0.75001.0
0.009(1)S40.25000.1645(3)0.6562(1)1.00.008(1)S50.50000.3301(3)0.5496(1)1.00.012(1)S60.50000.0828(2)0.5484(1)1.00.014(1)S7
0.2515(4)0.4154(2)0.6477(1)1.00.010(1)S8
0.2499(4)Note: Standard errors are in parentheses and represent the last significant
numbers.

DE VIllIERS ET Al.: CRYSTAl STRUCTURE OF A 5C PYRRhOTITE
elongation (PQE) as defined by Robinson et al. (1971) for the different octahedra
surrounding the Fe atoms. These were calculated by the program XtalDraw (Downs
and hall-Wallace 2003).
Refinement in three subgroups was then attempted and tested using thehamilton significance test (hamilton 1965). Only these three were tested, as

Table 3.

Anisotropic displacement parameters (Å2 × 103) for pyrrhotite-5C

U11
U22
U33
U23
U13
U12
Fe1
16(2)
36(2)
5(2)
–6(2)
0
0
Ni1
19(1)
11(1)
–4(1)
–3(1)
–11(1)
Fe2
26(1)
20(1)
12(1)
5(1)
0
0
Fe3
23(1)
15(1)
3(1)
0(1)
0
0
Fe4
30(1)
16(1)
12(1)
6(1)
0
0
Fe5
36(2)
9(1)
9(1)
–5(1)
0
0
Fe6
35(2)
26(1)
15(1)
2(1)
0(1)
2(1)
Fe7
10(1)
25(1)
18(1)
–4(1)
–1(1)
6(1)
Fe8
10(1)
9(2)
16(2)
–5(2)
0
0
S1
12(2)
9(2)
9(2)
–1(1)
0
0
S2
6(2)
9(2)
11(2)
1(1)
0
0
S3
5(2)
11(2)
12(2)
0
–1(1)
0
S4
4(2)
9(2)
7(2)
–2(1)
0
0
S5
9(2)
8(2)
14(2)
–5(2)
0
0
S6
12(2)
10(1)
20(2)
2(1)
0(1)
1(1)
S7
11(1)
10(1)
16(1)
3(1)
1(1)
0(1)
S8
6(1)
Note: The anisotropic displacement factor exponent takes the form: –2π2[ h2a*2U11+ ... + 2 h k a* b* U12].

Table 4.
Atom
Fe1

Metal-sulfur bond lengths (in angstroms) of the octahedra
in 5C pyrrhotite
Bond
S12
S74
Fe42

Avg. M-S
OAV
PQE
Fe4

Avg. M-S
OAV
OQE

Lengths
2.524(4)2.415(3)2.887(2)Atom BondFe2 S12
S62
S72
Fe72

2.453
1.521
1.00082.451
1.188
1.0012
S1
S5
S72
S82
Fe1
Fe5

2.562(5)2.400(4)2.419(3)2.482(3)2.887(2)2.887(4)2.461
23.993
1.0072

Lengths
2.502(3)2.428(3)2.430(3)2.914(2)

Fe5

S3
S42
S5
S82
Fe3
Fe4

2.361(4)2.499(3)2.488(4)2.446(3)2.908(3)2.887(4)2.457
17.5471.0053

Atom Bond Lengths
Fe3 S2 2.442(4)S3 2.428(4)S42 2.484(2)S82 2.452(3)Fe5 2.908(3)Fe6 2.953(4)
2.457
1.780
1.0006
Fe6

S2
S6
S72
S82
Fe3

Table 5.

lower-symmetry subgroups would necessitate refinement of very large numbers ofparameters. The results of the refinements of the subgroups are listed in Table 5.
Although significant improvements resulted, especially for space group Cm2a,none of the refinements resulted in reasonable atomic displacement parameters,
and restraining these for some atoms using the ISOR command in ShElX also
resulted in very distorted ellipsoids. The structure with space group Cmce istherefore considered to be the best candidate to describe the structure and vacancy
distribution in 5C pyrrhotite.
It is also necessary to comment on the relatively high R factor of 0.072 andGOF of 1.225, and whether the structure described here is the correct one. If thisstructure is compared with the structure of monoclinic pyrrhotite described by
Tokonami et al. (1974), the latter yielded an R of 0.093 for observed data and agoodness of fit of 1.69. Also, no anisotropic refinement was possible. Second,the overall structure of 5C pyrrhotite (positions of Fe and S atoms) is reasonablewith realistic bond lengths and angles (Table 4). It is therefore quite compatible
with the existing structure models of pyrrhotite. Third, refinement resulted in a
calculated composition, which is reasonable for this structure type, and comparesreasonably well with the microprobe analysis. The fact that partially occupied
sites project directly above each other in 5C pyrrhotite was also observed by Kotoet al. (1975) in their analysis of Fe11S12 and by Yamamoto and Nakazawa (1982)
in their study of Fe10S11.
Finally, hand-sorted fragments of pyrrhotite were ground with a mortar
and pestle, deposited on a low-background sample holder and examined with a
PANalytical X’Pert PRO diffractometer equipped with an X’celerator detector andvariable incident and diffracted beam slits. Refinement using CoKα was done using
the commercial TOPAS software (Bruker 2005).

desCRiPtion oF the stRuCtuRe
The structure is composed of hexagonally close-packed layers of S atoms octahedrally arranged around Fe atoms. Thesearrays form layers that are stacked perpendicular to the c axis.
Figure 2 shows the atomic arrangement of the layer containing
the vacant sites at z = 0.
The next layer of S and Fe atoms is arranged similarly to thefirst one, but with all Fe sites fully or partially filled (Fig. 3). The
upper layer of S atoms in Figure 2 now comprises the lower layerof S atoms in the layer shown in Figure 3. The resultant stacking

2.510(5)2.393(5)2.401(3)2.503(3)2.953(4)
2.452
5.060
1.00192.580(3)Fe8S2
2.462(4)2.477(3)S32.352(4)2.388(3)S32.438(3)2.414(4)S42.524(4)2.416(3)S5
2.517(3)2.486(3)S8
2.445(4)2.914(2)Fe7 2.886(4)2.886(3)
Fe8 2.788(4)
2.460
2.456
Avg. M-S
19.898OAV
24.8741.0062OQE
1.0076
Notes: The multiplicities of the bonds are given as superscripts, and the standard
errors are in parentheses and represent the last significant numbers. The octahedral angular variance is in degrees squared and the polyhedral quadratic elongation is dimensionless. The shortest metal-metal distances are also given.
Fe7

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S1
S2
S5
S6
S7
S8
Fe2
Fe8

FiguRe 2. Octahedral arrangement of S atoms around the layer of
Fe atoms at the origin of the unit cell (z = 0). The vacant cation sites are
situated at the centers of the unit-cell edges. The lower layer of S atoms
are labeled as l and the upper level as U.

Refinements of some subgroups of space group Cmce

Space group
R value
Dimension
Degrees of freedom
R-factor ratio
Hamilton test (0.5%)
Anisotropic ADP
Cmce
0.072
0
802
Pos. definite
Cmc21
0.061
98
1368
1.178
1.046
Non-pos def.
C2cb
0.059
69
1316
1.230
1.037
Non-pos def.
Cm2a
0.058
97
1204
1.248
1.052
Non-pos def.
Note: Although all refinements resulted in lower R values and significant improvements according to the Hamilton test (R-factor ratio > Test) at the 0.5% significancelevel, none resulted in reasonable atomic displacement parameters.

1408

DE VIllIERS ET Al.: CRYSTAl STRUCTURE OF A 5C PYRRhOTITE

arrangement comprises face-sharing octahedra stacked on topof each other. This face-sharing results in short Fe-Fe distances
listed in Table 4, e.g., between Fe1 and Fe4, Fe4 and Fe5, etc.
The shortest Fe-Fe distances (2.886 Å) are between thoseatoms that project above each other, i.e., Fe4 and Fe5, Fe7 andFe8. These are shorter than the values of 2.919, 2.947, and 2.984
Å, etc., given for troilite (Evans 1970). The face-shared arrangement of adjacent octahedra is shown in Figure 4, between thelower Fe7 and upper Fe8 octahedra.
The distribution of vacant and partially filled Fe sites in 5C
pyrrhotite is shown in Figure 5, and this is compared with the
distribution of vacant sites in monoclinic pyrrhotite, describedby Tokonami et al. (1972). In this setting, the structure of themonoclinic pyrrhotite can be directly compared with that of 5C
pyrrhotite. This choice of common unit cell facilitates comparison between the 5C and 4C pyrrhotites. This is also the cell of
choice of Wang and Salveson (2005) in their review. It must
be emphasized that the cell for monoclinic pyrrhotite shown in
Figure 5 is not the true unit cell, but one formed by connecting
the Fe atoms in the layers as shown.
The space group symmetry constrains the partially occupied
sites (Fe6) to project on either side of the vacant sites (layers 0
and 5). The partially occupied sites with Fe8 in layers 2 and 3 aswell in layers 7 and 8 project directly on top of each other. Thereseems to be a tendency for the partially filled sites to cluster
together. This was also described by Koto et al. (1975) in theirdescription of the vacancy arrangement of Fe11S12. In the latter
case, two half-filled Fe sites project directly on top of each other.
This was ascribed to the formation of a 1:1 intergrowth of defective (vacancy-containing) and filled layers. Such an intergrowth
could not be constructed for the structure described here.
The occupancy of Fe in the different layers in the 5C pyrrhotite is shown in Figure 6a, compared to that of 4C pyrrhotite.
A layer with all cation sites occupied by Fe atoms will have an
occupancy of 1, and the occupancy of a layer with two vacantsites is 0.75. In 4C pyrrhotite, alternating layers contain either

FiguRe 3. The atomic arrangement in the next layer of Fe and S
atoms. The Fe8 atoms project directly on top of the almost vacant Fe9
sites, and they have an occupancy of 0.86.

FiguRe 4. Adjacentface-sharing octahedra in
5C pyrrhotite, resulting inshort Fe7-Fe8 distances.

FiguRe 5. Distribution ofvacant and partially occupied
sites in 5C pyrrhotite (leftdiagram) as compared to
the vacancy distribution inmonoclinic pyrrhotite (right).
The layers are labeled from 0 to
9 for 5C pyrrhotite and from 0
to 7 in 4C pyrrhotite. The sites
with the partial occupancies
are shown in different shadesof gray. layers 1, 3, 5, and7 in 4C pyrrhotite are also
described as the A, B, C, andD configurations.

DE VIllIERS ET Al.: CRYSTAl STRUCTURE OF A 5C PYRRhOTITE

1409

FiguRe 6. (a) Occupancy of Fe in the different layers of 5C pyrrhotite. The minimum occupancy of Fe is 0.75 in layers 0 and 5; the maximum
is 0.968 in layers 1, 4, 6, and 9. The occupancy in layers 2, 3, 7, and 8 is 0.893. The average occupancy is 0.908. layers 0 and 10 denote theborders of the unit cell. (b) Occupancy of Fe in monoclinic 4C pyrrhotite. This varies periodically between 1 and 0.75. layers 0 and 8 denote theboundaries of the unit cell.

fully occupied layers or layers with 2 vacant sites out of a possible 8, giving the layer an occupancy of 0.75.
So, the occupancy of monoclinic pyrrhotite will show asawtooth pattern with alternating maxima at 1 and minima at
0.75 as shown in Figure 6b. In 5C pyrrhotite, the occupancy invacancy-containing layers 0 and 5 is 0.75; in layers 2, 3, 7, and8, it is 0.893; and in layers 1, 4, 6, and 9, it is 0.968.

disCussion
The structure of the 5C pyrrhotite has been discussed by various authors. Vaughan et al. (1971) described the mineral fromthe Strathcona mine, Sudbury, as hexagonal and postulated thestructure as comprising single and double filled layers alternating with layers containing vacancies. Koto et al. (1975) in theiranalysis of the superstructure of a 6C pyrrhotite postulated anarrangement of filled and defect layers, where the occupancy of
the Fe atoms in the defect layers is one half. This means that thestructure is explained not by the ordering of filled and vacantFe sites, but by the arrangement of adjacent partially occupied
sites. Unfortunately, however, they do not give a full structure
determination for 6C pyrrhotite with unit cell, space group, andatomic positions.
The structure described here consists of two layers containing vacancies and eight layers containing partial occupancies.
This can be compared to the structure postulated by Yamamoto
and Nakazawa (1982) using a superspace formulation. They describe the 5C structure as consisting of two layers with adjacentpartially occupied sites alternating with three layers of adjacentpartially occupied sites. This can be schematically described as
AABBBCCDDD with the sites partially occupied. In the caseof the 5C structure described here, the distribution could be
described as AA′A(BD)CC′C(BD) where A′ and C′ are layers
containing vacancies and (BD) are layers that contain partiallyvacant sites in both the B and D configurations. The A, B, C, andD configurations correspond to the 1, 3, 5, and 7 layers in the 4C
pyrrhotite in Figure 5. The structure therefore also consists of
two layers alternating with three layers of partially occupied or
vacant sites projecting above each other. The structure containing
an additional partially occupied Fe site at (0,0,½) can therefore
not be discounted entirely. This will have to be resolved if better
crystals become available.
The non-stoichiometry most likely arises from the random
stacking of layers containing vacancies with layers containing
filled sites, giving rise to average structures with partially occupied sites. The partially occupied or vacant metal sites present
in this structure can therefore be stacked in various proportions
to give rise to a continuous variation in metal-to-sulfur ratios
within the compositional limits of this structure type.
The superspace model of Yamamoto and Nakazawa (1982)
was further refined by Izaola et al. (2007) who described thepyrrhotites in terms of the ordering of four different vacancy
Fe layers and two different S layers. A new refinement of theincommensurate Fe10S11 (or Fe0.91S) is presented by them. The
use of this formulation to describe the partial occupancies ofall pyrrhotites as a group is obviously desirable and needs to beinvestigated further.
The structural description given here can therefore be shownas giving the correct composition of the 5C structure type. The
calculated powder XRD pattern is compared with that of monoclinic pyrrhotite in Figure 7. The single strongest peak of the 5C
pyrrhotite is compared with that of the double peak of the 4C
variety. This confirms the interpretation of Graham (1969).
A Rietveld refinement of hand-picked crystals of the copper
Cliff North material achieved excellent correspondence between
observed and calculated intensities with a Rwp of 1.7% and a RBraggof 1.15% with a Goodness of Fit parameter of 1.41.

1410

DE VIllIERS ET Al.: CRYSTAl STRUCTURE OF A 5C PYRRhOTITEFiguRe 7. Calculated X-ray powder diffraction pattern showing
the single peaks of the 5C pyrrhotite compared with the double peaksof monoclinic 4C pyrrhotite based on the structure of Tokonami etal. (1972).

The mechanism and reason for the non-stoichiometry is
still a matter of dispute. A simple model that explains themagnetic properties of Fe7S8 requires the presence of Fe3+ inthe vacancy layer. If four Fe2+ atoms are present on the fully
occupied planes and one Fe2+ and two Fe3+ atoms on the planes
containing vacancies, then the correct net magnetization of 2µB
is obtained. This model also explains the non-stoichiometry of
the various pyrrhotites by a simple charge-balance mechanism,i.e., substitution of Fe3+ for Fe2+, resulting in a deficiency of Fe tomaintain electrical neutrality. The formula of Fe7S8 can therefore
3+
2+
3+
be written as Fe2+
5 Fe2 S8 and that for Fe9S10 as Fe7 Fe2 S10. The
existence of both ferrous and ferric iron in monoclinic pyrrhotite
was confirmed by Pratt et al. (1994) using X-ray photoelectron
spectroscopy. On the other hand, no evidence of Fe3+ could befound by Mössbauer spectroscopy, (Oddou et al. 1992). Also,in a recent study by letard et al. (2007) using X-ray magnetic
dichroism, no evidence of ferric iron could be found in Fe7S8
pyrrhotite. If this is the case, then a more complicated mechanism
is responsible for the non-stoichiometry, and for the stability ofpyrrhotite relative to troilite. Clearly, this is an area that needs
to be investigated.

aCknowledgMents
Microprobe analysis of the pyrrhotite single crystal by P. Gräser is gratefully
acknowledged. Thanks also to Alison Tuling for help with the diagrams.

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Manuscript received august 27, 2008
Manuscript accepted May 13, 2009
Manuscript handled by laurence garvie

note added in PRooF
Subsequent to the submission of this paper, a second suitable
crystal was found, and its X-ray data refined. The resultant Rfactor was R = 0.06, and the unit-cell and structure parameters
were very similar to the values given here. These are available
from the author on request.