Synthesis and Rietveld crystal structurerefinement of mackinawite, tetragonal FeS
A. R. LENNIE*, s. A. T. REDFERNt, P. F. SCHOAELD~ AND D. J. V AUGHAN** Department of Earth Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
tDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
tDepartment of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK

Abstract
Mackinawite,tetragonal FeS, has been synthesised by reacting iron with Na2S solutions. A Rietveld structurerefinement of X-ray powder diffractipn data, recorded using X-rays monochromated from synchrotron
radiationwith a wavelength of 0.6023 A, has been performed. The structure has been refined in the tetragonalsJ'.!lcegroup, P4/nmm, and has the following cell parameters:

a = 3.6735(4), c = 5.0328(7) A, V = 67.914(24)A3.Our refinement shows that the FeS4 tetrahedron in mackinawite is almost perfectly regular, with a muchsmallerdistortion than has been previously reported. An improved X-ray diffraction data set is provided.
KEYWORDS:mackinawite,

Rietveld

structurerefinement,

Introduction

THEmineral mackinawite, first described by Evans etal. (1962, 1964), is identical to synthetic tetragonaliron monosulphide(Berner, 1962,1964),an unnamed
tetragonaliron sulphide mineral (Kuovo et al., 1963)
andthe synthetic iron sulphide 'kansite' (Meyer etal., 1958). The importance of this metastable(Davison, 1991) iron-sulphur phase is due to itsrole as precursor to the formation of pyrite insedimentary and hydrothermal systems.
Mackinawite possesses a layer structure similar tothat found in tetragonal PbO (litharge, space groupP4/nmm),with Fe and S occupying the sites of 0 andPb, respectively, in litharge. X-ray diffraction
patterns from natural mackinawite (Kuovo et al.,1963),and neutron (Bertaut et al., 1965) and electron(Uda, 1968) diffraction analyses of synthetic
mackinawite, support this interpretation.
A structural refinement of a natural crystal ofmackinawite from Outukumpu, Finland (Taylor andFinger, 1970), shows that the structure may bethoughtof in terms of a distorted cubic close-packed
array of S atoms with some of the tetrahedral
interstices filled with Fe. Each metal atom is at thecentre of a slightly distorted tetrahedron of S atoms.
These tetrahedra share edges forming sheets ofsynthesis.

tetrahedra which are stacked normal to the c-axis,
with only weak van der Waals forces holding thesesheets together. Mackinawite has been reported asboth a metal-excess (Clark, 1966) and a sulphur
deficient (Taylor and Finger, 1970) phase.
Tetragonal Fel+xSe and Fel+xTe, which share theanti-litharge structure found in mackinawite, arereported as metal-excess chalcogenides (Tsuji et al.,1976); a compilation of data on these phases is given
in Table 1.
In addition to the tetragonal iron chalcogenide
phases, it is of interest to note that there exists afamily of AB2X2 compounds (Hoffmann, 1988)
having the ThCr2Sirtype structure consisting of
B2X~- layers separated by A 2+ layers. The bonding
between A2+ and B2X~- layers appears largely ionic,
but within each B2X~- layer there is not only B-X
bonding, but also an indication of some metal-metal
B-B bonding. These B2X~- layers share the samestructure as the sheets of FeS4 tetrahedra which makeup mackinawite.
Several methods for synthesising mackinawite aredescribed in the literature, including routes involving
precipitation of Fe2+(aq)by H2S, reaction of metallic
Fe with H2S in solution (Berner, 1964), andelectrochemical formation (Yamaguchi and Moori,1972). Dry synthesis of mackinawite (Takeno et al.,Mineralogical Magazine, December, Vol. 59, pp. 677-683@ Copyright the Mineralogical Society

678

A. R. LENNIE ET AL.

TABLE 1. Properties

Iron chalcogenide
Maximum
temperature ofstability
Unit celldimensions
at 298 K (A)Density
Referencesof iron chalcogenides

with tetragonal-PbO(space group P4lnmm, Z

Fel+xS(x = 0.00-0.07)

Fel+xSe(x = 0.05)

540 K(to NiAs-type)731 K(to NiAs-type)

1200 K
decomposes

a = 3.6795(8)a = 3.77==

a = 3.83
c = 6.27
cia = 1.637
6.85 g cm-3
(7,8)

c = 5.030(2)1.3670(7)cia4.28 g.cm-3(1,2,3)(1) Taylor and Finger (1970)(3) Vaughan and Craig (1978)(5) Gr~nvold and Westrum (1959)(7) Haraldsen et al. (1944)

1982) requires addition of substantial quantities of
Cu, Co or Ni; synthesis of mackinawite fromelemental Fe and S has not been achieved.
A conflicting range of values is reported for theupper temperature limit of stability for mackinawite.
Kuovo et at. (1963) report that a reconstructive
transition from mackinawite to hexagonal pyrrhotite(FeS) occurs at 518 K, while Sarkar (1971) reports atemperature of between 493 to 498 K for the sametransition. In contrast, stability limits in the much
lower temperature range of 393 to 426 K have beenreported (Takeno and Clark, 1967; Takeno, I965a,b;
Clark, 1966). These low breakdown temperatures ofnatural mackinawites may, however, involve additionof sulphur from surrounding minerals (Zoka et at.,1972), and would not, therefore, be expected to be
relevant to the closed system.
The Rietveld refinement and synthesis experimentdescribed here forms part of a larger study (Lennie,
1994) which provides new information on thethermal stability of mackinawite, the expansion of
the unit cell with temperature, and on solid-statetransformations of this phase to both hexagonal
pyrrhotite (Fel_xS) and to greigite (Fe3S4)' the thiospinel of iron.

Experimental

structures

synthesis of mackinawite

Thirty cm3 of a solution of NazS (20 g of NazS'9HzO
in 200 cm3 deionised water) was added slowly to 500
cm3 of an 0.5 molar acetic acid/acetate buffer (pH =4.6) in which approximately 4-5 g of finely divided
iron wire (Roncraft, Sheffield) had been previously
immersed. Dissolution of the iron wire by reaction=2)

Fel+xTe(x = 0.11-0.25)

c = 5.52
cia
1.464
5.71 g cm-3
(4,5,6)(2) Lennie (1994)(4) Hagg and Kindstrom (1933)(6) Gr~nvold (1968)(8) Gr~nvold et al. (1954)

with the acetate buffer for 30 minutes or so evolved
Hz, providing a reducing environment. The NazS
solution was then added slowly, and exceeded thecapacity of the acetate buffer, increasing the pH to avalue of approximately 6.5. This solution, containing
the iron wire, was allowed to stand open to theatmosphere at room temperature for 24 hours.
The iron wool generated Hz while dissolving, andthis gas, trapped in the wool matte, floated the ironto
the top of the solution. Mackinawite forming on theiron surface spalled off and dropped to the bottom of
the flask giving good (but not entirely complete)separation of mackinawite from the iron.
After reaction, the iron wool was removed and thesupernatent solution poured off. The mackinawite
remaining was rinsed onto a 0.45 lim filter, thenrinsed several times with deaerated deionised water
while a water vacuum was applied to remove thesolution. While still moist, the mackinawite wasplaced in V-shaped aluminium foil boats approximately 2 cm long. After crimping both ends of thefoil, these were placed in borosilicate tubes
previously sealed at one end using an OZ-CH4
flame. These tubes were partially prefilled withenough acetone to completely cover the mackinawite
contained in the aluminium boat. Surplus acetone
was then removed from the tube.
A number of tubes of mackinawite prepared in thisway were dried together in a flask for two hours
under a vacuum of IO-z mbar; acetone and water
removed from the sample was condensed in a liquid
Nz cooled trap. When dry, the tubes were removed
from the evacuated flask under an Oz-free Nzatmosphere, and individually sealed, under vacuum,using an O-CH4 flame.

MACKINA WITE SYNTHESIS AND STRUCTURE REFINEMENTTABLE2. Compositional

analyses of synthetic FeS

1. ICP analysis of mackinawite dissolved in HCIElement

wt.%

atom%

Fe
Ii
Mn

63.2
0.10
0.17

49.67
0.09
0.14

S (by difference)

36.53

50.10

2. X-ray fluorescenceanalysis of mackinawite

Element

wt.%

Cu
Zn
Ni
Mn
Cr
Ii

<0.001
0.005
0.023
0.39
0.034
0.16

Initial identification of mackinawite using X-ray
powder diffraction was carried out using a Philips Xray diffractometer, with Cu-Kex X-rays generated at
40 kV and 20 mA. Samples were mixed with amyl
acetate, spread on glass slides, and dried in air. The
diffraction pattern of mackinawite showed broad
peaks with the peak width at half peak height in theorder of 0.50 28; peak positions were consistent withpreviously reported diffraction patterns of mackinawite. The broad peak widths are due to the small
particle size of the mackinawite crystallites.
A sample was prepared for compositional analysis
by dissolving a weighed amount in hydrochloric acid.
After the majority of sulphide had been removed (asH2S) this solution was analysed by ICP spectrometry.
A second sample of mackinawite powder was
analysed by X-ray fluorescence to determine
concentrations of impurities in the iron sulphide.
The powder, pressed into a disc of boric acid at 5 tons
applied pressure, was analysed using both Cr and Rh
X-ray sources on a Philips PW 1450 XRF spectrometer. Both analyses are recorded in Table 2. The
XRF analysis shows metallic impurities amounting to
0.61 wt.% in FeS. No fluorescence peaks due to Se
were observed.
High-resolution

powder diffraction

The synthetic mackinawite was further characterised
by Rietveld structure refinement. High resolution
powder diffraction data were recorded at Station 9.1
on the 5 Tesla wiggler beam line of the Daresbury

Laboratory

Synchrotron

Monochromaticradiation

679

Radiationof wavelength

Source.
0.6023A

was selected from the channel-cut Si (111) monochromator, the radiation wavelength being calibrated
from the positions of six reflections of a Si NBS 640b
sample.
Mackinawite rapidly oxidises under ambient atmosphere conditions, and diffraction patterns were
therefore collected from samples loaded directlyfrom sealed borosilicate tubes under an inert atmosphere. This involved sealing the sample chamber ofthe diffractometer in a He-filled glove bag, breaking
open the borosilicate sample vial, and loading
mackinawite directly onto the aluminium flat platesample holder (15 mm diameter, 1 mm deep) of the
instrument. The sample chamber was continuallypurged with He during data collection. This mode ofsample loading, and the limited amounts of sample
available to us, necessarily compromised the qualityof sample presentation within the diffractometer. The
sample was not perfectly flat within the sample
holder, and electrostatic effects meant that the sample
could not be ground to as fine a powder as would
normally be preferred. Nonetheless, data of sufficientquality for satisfactory Rietveld refinement wereobtained.
Diffraction data were collected between 5 and
58028 (d = 6.91 to 0.622 A) at steps of 0.010 28.
Because mackinawite is extremely easily oxidized,
the X-ray wavelength was selected to be above the Feabsorption edge to reduce photo-oxidation effects.
No degradation of the sample was observed whenheld at room temperature in the He atmosphere; usinga shorter X-ray wavelength also has the advantage ofgiving measurements of smaller interplanar spacings.
Results and discussion
The diffraction pattern showed mackinawite as thedominant phase, although some diffraction from ex-Fe
was observed.
The regions
16.80-17.400,
24.0-24.60, and 29.5-30.1 ° 28, containing thediffraction peaks from ex-Fe, were thus excluded
from the Rietveld refinement. The diffraction patternused for the refinement is illustrated in Fig. I.
The raw diffraction pattern was prepared for
Rietveld refinement using the program MPREP,
and refined using MPROF in the Powder
Diffraction Program Library of Murray et ai.
(1990). The background was fitted with a polynomial
curve; an overall scale factor, zero point, andpreferred orientation factor (for orientation parallelto (001» were also used. An asymmetric pseudoVoigt line shape was employed. The structure
proposed by Uda (1968) (space group P4/nmm) wasused as a starting model. The refineable sample
parameters were, therefore, restricted to cell edges,

A. R. LENNIE ET AL.

680

25000

20000
r-....
'"~;j 150000u'-';>,.~~Q) 10000
'"

+

....
~.-

5000

0
I

II I

10

5

I

15

II II

II I

20

I

I II 11111 I

25

30

III III 1111 I I 111111I
I 1111111111111
III 11111 1111111111I1 111111

35

40

45

50

55

60

Degrees (28)
FIG. 1. X-ray diffraction (XRD) pattern of mackinawite obtained using synchrotron radiation. The difference
between observed and Rietveld calculated intensities is shown below the XRD pattern.

TABLE 3. Refined structure parameters for synthetic
mackinawite. Errors (a) are in parentheses
Space group

isotropic temperature factors, and the z-coordinate of
the S atom. The refinement converged to give thestructural parameters and R factors summarised inTable 3, where

P41nmm (No. 129) origin at 4m2
R[

Cell parameters(Rietveld)

Atomic positionalparameters
Fe in 2(a):S in 2(c):= 100.1: 1 lobs - lea1c 1 / 1:1 lobs 1
Rp = 100.1: I Yobs - Yeale 1 / 1:1 Yobs I]0.5
Re = 100. [ 1: [N - P] / 1:1 W. Yobs 1

a (A)
b (A)
c (A)
V(A3)

3.6735(4)3.6735(4)5.0328(7)67.914(24)

x
0
0

y

z

o
0.5

0.41(2)o0.2602(3) 0.20(2)The structure was refined with the following
R[=9.15%
Rp=12.42%

B(iso)

R factors:
Re=4.39%

lobs and lea1c are the observed and calculated
intensities for Bragg reflections, and Yobs and Yealc
are the observed and calculated intensities at eachpoint in the diffraction pattern. N is the number ofpoints in the pattern, P the number of parametersrefined ([N-P] = 4165 in this case), and w is theweight assigned to each step intensity. The relatively
high values of R[ and Rp reflect the small deviations
in sample height across the flat plate sample holder,which arose from the difficulties of sample loading.

MACKINA WITE SYNTHESIS AND STRUCTURE REFINEMENTTABLE4. Bond distances and angles for synthetic
mackinawite. Errors (0') are in parentheses
Bond distances (A)
Fe-S
Fe-Fe
Bond angles n
S-Fe-S

Fe-Fe-Fe
Fe-S-Fe

2.2558(9)2.5976(3)2 x 109.0251
2 x 109.6947
4 x 902 x 70.3053
2 x 109.0251

The agreement between the observed and calculated
profiles in the Rietveld difference plot is illustrated inFig. 1.
This structural analysis is consistent with previous
reports of the structure of mackinawite; we were notable to refine vacancy occupancy or surplus Fe
occupancy with any degree of significance. This
refinement does not, therefore, provide any further

681

information on non-stoichiometry, either in terms of
surplus Fe, or S-deficiency in the structure; asatisfactory refinement is achieved by assuming
stoichiometric tetragonal FeS.
Comparing our refinement with the mackinawite
structure of Uda (1968), we see that S is significantly
displaced from z = 0.25. This has an impact on the
resultant coordination environment (Table 4). Our
refined structure shows that the FeS4 tetrahedron isvery nearly perfectly regular, with two S-Fe-S bondangles which deviate only very slightly from the ideal
tetrahedral bond angle. Each S is bonded to four Fe in
an asymmetric one-sided four-fold coordination with
Fe-S-Fe bond angles of70° and 1090, while the short
Fe-Fe distance of 2.5976 A (Fe in square planar
coordination by Fe) confirms the importance of FeFe metallic bonding within the (001) layers (Fig. 2).
A listing of the hkl Bragg reflections withassociated interplanar spacings, and observed andcalculated intensities obtained from this experiment,
is given in Table 5. When compared with previously
reported data for this mineral (Berner, 1964; Uda,
1968), this tabulation provides a more extensive
range of experimental hkl intensity values, giving
intensities of Bragg reflections out to 0.68 A.

P-y
y

z

FIG. 2. Refined structure of mackinawite projected parallel to [001] (left) and [100] (right). Small filled spheres
represent Fe atoms; large spheres represent S atoms. Fe-S bonds are shown, together with the outline of the unit cell.
The structure comprises (001) sheets of edge-sharing FeS4 tetrahedra, with van der Waals bonding between sheets.

A. R. LENNIE ET AL.

682
TABLE 5. Powder
parameters

diffraction

pattern

for mackinawite

with intensities

calculatedusing Rietveld

refined

hkl

d (A)

lcalc

lobs

hkl

d (A)

lcalc

lobs

001
101
110
002
111

5.03282.96722.59762.51642.3082

100
38
8
4
64

100
50
8
7
70

402
215
314
331
412

0.86270.85830.85350.85330.8399

<1
2
<1
2
<1

<1
2
1
2
<1

102
200
112
201
003

2.07601.83681.80741.7254
1.6776

<I
37
69
23
7

<I
38
82
29
9

006
420
332
106
421

0.83880.82140.81870.81780.8107

<1
4
4
<1
3

<I
5
5
<1
3

211
103
202
212
220

1.5617
1.52601.48361.3756
1.2988

11
5
3
<1
12

12
6
4
<1
11

403
116
225
324
413

0.8056
0.79820.7956
0.79180.7869

2
4
1
<1
1

2
4
1
<1
1

004
221
203
104
301

1.2582
1.25761.23871.1903
1.1898

5
8
10
<1
2

6
9
10
<1
3

422
305
333
206
315

0.78090.7776
0.76940.76300.7607

<1
1
1
<1
3

<1
1
2
<1
4

213
310
222
114
311

1.17381.1617
1.15411.1324
1.1319

5
1
1
1
12

6
1
1
1
14

216
404
423
414
501

0.74710.74180.73770.72710.7270

<1
3
2
<1
<1

d

302
312
204
223
005

1.10111.05471.03801.02701.0066

<1
23
10
5
1

<1
26
12
6
I

431
510
007
325
334

0.72700.72040.71900.71600.7133

1
<1
<1
1
<1

1
<I
<I
1
<I

214
321
303
105
313

0.99890.9986
0.98910.97080.9550

<1
3
1
I
4

<1
4
2
2
6

511
107
502
432
226

0.71320.70560.70530.70530.7046

2
<1
<1
<1
<1

2
d
d
<I
d

322
115
400
224
401

0.94440.9386
0.91840.90370.9035

<1
4
3
6
2

<I
4
4
8
3

117
512
306
424
316

0.69290.69260.69200.68780.6800

<1
4
<1
4
4

d

205
304
411
323
330

0.88270.87750.87730.87080.8659

I
<1
2
2
<I

2
<1
2
2
<1

2
3
d
1

5
d
4
3

MACKIN

A WITESYNTHESISAND STRUCTURE

Acknowledgements
Paul Lithgoe and Tim Jenson are thanked foranalyses of mackinawite. Graham Bushnell-Wye
and Tony Bell provided valuable assistance withcollection of powder diffraction data and the Rietveld
refinement respectively. This research was supported
by NERC, and by a SERC small grant (number 22/258) for synchrotron studies.
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[Manuscript received 24 January 1995:
revised 6 April 1995]