American Mineralosistvol .57, pp. 1066-1080 (1972)

CRYSTAL STRUCTURE OF A MONOCLINICPYRRHOTITE (FezSs)

Mesev,s.su TonoNAlrr,l K.q.rsuHrse Nrsrrrcucur' aNl NosuoMoRtr,toto,s Institute of Scientifi,c and Industrial Research,

Osaka Uni"uersity, Suita, Osaha 565, Japan.

Assrnecr

The crystal stmcture of the monoilinic 4C-type pyrrhotite (Fe'S") has beenrefined using the three-dimensional intensity data collected wrth Zr filtered MoK"radiation from an untwinned crystal from Kisbanya, Rumania. This crystal hasthe cell dimensions of a = 11.902 -{- 0.008, b = 6859 -F 0.005, c - 22.187 + 0.010A, andp - 90"26'-{- 3'. It was refined in space group FZ/d and the final weighted-R factor is 0.045 for 487 crystallographically independent reflections.

The structure obtained confirms the ordered arrangement of vacant positionsfor iron atoms proposed by Bertaut (1953), and gives shifts of iron and sulfuratoms from the atomic positions in the ideal NiAs-type structure. All iron atomslie between six sulfur atoms with octahedral arrangements and also between a fewiron atoms which must be considered to be the nea.rest neighbors because ofshort Fe-Fe distances. The mean Fe-S bond lengths in the four FeSc octahedraare 2.446, 2.456,2449, and,2.444 A, and the Fe-Fe distances are 2868, 2.911, and2.956 A along the c axis. In the iron layer normal to the c axis, one of the Fe-Fedistance is 2944 A, though the others are more than 3.1 A. The group of seveniron atom3 along the c axis can be considered as a collective unit in the 4C-typepyrrhotite as the triangular group in the troilite structure. Each iron group isconnected with two others through the short Fe-Fe bonds in the basal ironlayers.

T'he atomic arrangements around the vacant positions for iron atoms showslight contraction of the vacant space in comparison with the space around ironatoms.

INrnooucrroN

Pyrrhotite (Fer-"S) has been extensively studied, because of the

importance and complexity of its phase relations, structures, andmagnetic properties. A detailed review on the studies of the propertiesand structures of pyrrhotite has been given by Ward (1970). Fivedifferent types of pyrrhotite have been conflrmed to be stable at roomtemperature by studying single crystals of natural pyrrhotite (Mori-

moto, Nakazawa, Nishiguchi, and Tokonami, 1970; Morimoto, Naka-

lPresent address: Fachbereich Geowissenschaften; Mineralogie, Kristallog-raphie, Petrologie, Philipps-Universiilit, 355 Marburg/Lahn, Deutschhausstr. 10,Germany.

'Present address: Industrial Research Institute of Chiba Prefecture, Chiba280, Japan.

" To whom all the corresnondence should be addressed.

1066

MONOCLINIC PYRNHOTITE 1067

zawa, Tokonami, and Nishiguchi, 1971). The flve types are super-structures of the NiAs-type structure, with cell dimensions ,4., about3.45 A; C about,5.8 A; and have essentially stoichiometric composi-tions, Fe,-1S, (n ) 8), with the structures of (n/2)C type for n evenand of nC Lype for n odd (Morimoto, Nakazawa, Nishiguchi, andTokonami, 1970). Three new types of pyrrhotite stable at high tem-peratures were also described by Nakazawa and Morimoto (1971).

Monoclinic pyrrhotite of the 4C-type superstructure occurs innature at and near the composition FerSr, with a : 213A, b : 2A,and c : 4C, and 0 : 90.4o. On the basis of geometry of the diffractionpatterns of trvinned crystals of this pyrrhotite, Bertaut (1953) proposedthat the superstructure contained ordered vacancies in alternate ironlayers normal to the c axis. The space group and cell dimensions deter-mined by him, were recently confirmed by Corlett (1968) and Wuensch(Ward, 1970) using untwinned crystals. However, there was no report onthe crystal structure of the monoclinic 4C-type pyrrhotitebased on thecomparison of the observed and calculated structure factors, thoughthe structure of troilite (FeS) was determined by Bertaut (f956) andrefined by Evans (1970) and that of the hexagonal 3C-type was deter-mined by Fleet (1971).

The present investigation has been initiated to describe the crystalstructure of the monoclinic 4Ctype pyrrhotiie using the three-dimen-sional counter-measured intensities and to elucidate the arrangementof vacant positions for iron atoms and the effects of the vacancies onthe shifts of iron and sulfur atoms from the ideal NiAs structure. Thecharacteristics of complex magnetic and electric properties of thepyrrhotite can be explained based only on the precise crystal structure.

ExpontlrpNt,u

Many crystals of monoclinic pyrrhotite from various localities were examinedby the X-ray single crystal method to find single crystals for structure determi-nation. The crystals examined were in twin relations by 60' or 120' rotationabout the pseudo-hexagonal c* axis. Cr5'g1r1* from Kisbanya, Rumania were alsoexamined, which had been described by Corlett (1968) and given to us by her.Though the crystals also showed the twin relations, a small platy fragment with0.03 X 0.03 X 0.01 mm was found to be untwinned, and used for intensity collec-tion throughout in this study. AII attempts to obtain larger single crystals thanthis specimen were not successful.

If we assume symmetry centers in the structure of the pyrrhotite, the possiblespace group is C2/c, which was conveniently expressed as F2/d'by Bertaut (1953)to keep an approximate orthogonal relation between the axes. This centrosymmetricspace group was adopted during the structure determination and successful refine-ment of the structure confirmed this choice. The ceII dimensions were determinedfrom the goniometry of the reciprocal Iattice points 12.4.0, 6.6.0, and 0.0.32 usingthe Rigaku four-circle automatic diffractometer with Mo-Kor radiation (I : 0.70929

1068 TOKONAMI, NISHIGUCHI, AND MORIMOTO

,E). Based on space grortp F2/d,, they are a : 1L.902 + 0.008, b : 6.859 + 0.005,c : 22.787 + 0.010 A, and F : 90o26' :t 3' in good agreement with the valuesobtained previously (Corlett, 1968; Wuensch, 1969). The estimated standard devia-tions of the cell dimensions were obtained by the relative errors derived from theequation, 2d, sir' 0 : /rtr, considering possible errors in reading of angles. The calcu-Iated density of this specimen is 4.62 B.cm-: for eight FezSa units in the unit ceII,which is in good agreement with the observed value of 4.56 g'cm-a (Palache,Berman, and Frondel, 1955).

The integrated intensities were measured using the automatic diffractometerwith Zr-filtered MoKa radiation (0.7107 A). The intensities of 487 crystallographi-cally independent reflections were measured within the range of sin a/I ( 0.5 bythe e20 scan technique. The specimen was so small that 75 reflections had theintensities equal to or less than the background values, and were regarded to bezero in intensity. Reflections with sin 0/x > 0.5 were not measured. The intensitydata were converted into observed str-ucture factors by applying the standardLorcntz and polarization corrections. The absorption correction was not made,because the linear absorption coefficient was 125 cm-' and the maximum Iength ofthe specimen was 0.003 cm.

Srnucrunn DprnnrurNerroN eNr Rprrununxt

Bertaut (1953) proposed a crystal structure for the C-lype monoclinic pyr-rhotite based on the space group FZ/d (Fig. 1), but he did not give explicitly anyatomic coordinates of the structure. In order to describe the atomic coordinates,the space group F2/d is maintained and expressed by the following coordinatesystem (Fig. 2) throughout in this investigation:

origin at T on a 'diamond' glide plane d with a translation of (a I c) /a;unique axis b;coordinates of equivalent positions:

(0, 0, 0; t , * , 0; i ,0, L;0, 4, \ ) *r, !1, z', L - r, A, L - zl i, !, 2; i I r, U, i I z.

The geometrical structure factors for this system are as follows:

[ h+ t :+ "l h + l t :2n

A : 16 cos 2r(hr * le) cos 2rky, B : 0;

A : -16 sln?n(hx f lz) sin 2rhy,B : 0; and: 4 n + 2: 2 n

: 2 n * I

o r A : B : 0: 2 n * l

According to this system, the atomic coordina,tes of iron atoms determined byBert"aut are described as in Table I by fixing one of the vacant positions at 1,/8,l/8,7/8 (Fig. 1). The iron atoms in the filled layers are represented in this inves-tigation by odd numbers [Fe(l) and Fe(3)] and those in the vacancy-containinglayers are by even numbers [Fe(2) and Fe(4)]. For this a.rrangement of ironatoms, two alterrative arrangements of sulfur atoms are possible, because thereare two possible positions for sulfur layers in the closest packing relation with theiron layers. The atomic coordinates of sulfur atoms in one of the arrangements

{ h + tl h + h

fn+t1o *,

MONOCLINIC PYRRHOTITE

Frc. 1. The ideal structure of ihe 4C-type pyrrhotite proposed by Bertaut(1953) based on space $oup FZ/d. Only iron layers are indicated and sulfur layersare omitted for simplicity. Squa,res represent vacant sites.

1069

c

TOKONAMI, NISHIGUCHI, AND MORIMOTO

Frc. 2. Space group sJrmmetry and equivalent positions of F2/d.

a,re listed in Table 1, while those in the other can be obtained by replacing (r,U, z) for sulfur atoms by Q/A - n, a, e) in Table 1. The choice between thesetwo arrangements was carried out by comparison of F, and F" for hkl withh: !2 (mod 6) and I - -+-4 (mod 16), because the contributions of sulfuratoms to .F, of these reflections are same in absolute value but opposite in signfor these two altematives. Because the atomic coordinates of zulfur atoms listedin Table I gave better agreement between l', and F, for the reflections men-tioned above, they were determined to be correct ones for this stmcture.

tr'ull matrix least-squares refinements of the structure were carried out on theFACOM 230-60 of Kyoto University with the use of RStr'LS-4 of the UNICSsystem (universal crystallographic computation system, crystallographic societyof Japan, 1967) modified by Sakurai from ORFLS written by Busing, Martin, and

tab le 1 . A tomic coord ina tEs o f the 4C- type pyr lho t i re (Fe?58)

based on the idea l s t ruc tu re p roposeCl by Ber tau t ( I953) .

r070

Number o fequ iva leDt po in ts

S i t es)metry x y

PeIFe2

Fe4S Is 2

s4

I 6l 61 6

I

1 51 6I 5

tLI21l-tI

3/e 3/8 03/8 3/8 t /83/8 3/8 2/83/8 3/8 3/8s / 2 4 3 / 8 - t / L 63/24 3/8 I /16s/24 3/A 3/163/24 3/e 5/!6

MON OC LIN IC P'' RRHOT IT E 1071

Levy (1962). The function minimized is 1/(o)" l l1, l - l f ' . l l ' , where o representsthe estimated standard deviation computed from counting statistics' The esti-mated standard deviations of the weakest non-zero reflections were uscd for thereflections which were regarded. as zero intensity. Scattering factors were takenfrom MacGillavry and Rieck (1962), assuming that sulfur atoms are doublyionized and iron atoms are the mixture of 5/7 Fe'* and 2/7 Fe'*. Isotropic tem-perature factors were assumed as 1.0 for all atoms. Refinement was initiated usingthe atomic coordinates of the ideal stmcture (Table 1), with the arrangement ofsul{ur atoms described above, which gave ,R - 0.395.

The full matrix least-squares failed by divergence of the E coordinates when thex, y, and a coordinates were allowed to vary together. The first two cycles were,therefore, carried out with the fixed g coordinates varying only the t and z co-ordinates. In the next two cycles, all the z, E, a coordinates were allowed to vary.The final four cycles were carried out varying the atomic coordinates and isotropictemperature factors. The final R factors for all reflections and for reflections excludingzero intensity are 0.134 and 0.093, respectively. The final weighted E factor was0.045. At this stage, the order of differences between F. ar'd F " have become almostequal to the order of oF, resulting in 1.69 for the quantity of error of fit,

[Ir" {(f" - F")16F.'l/(N" - ,liro;trz, where ff" is the number of observed reflec-tions and No the number of adjustable parameters. Any further refinement withincreased number of parameters is, therefore, likely to result in no significantimprovement. In fact, attempts to determine the ordering of ferric and ferrous ionsin the structure or the anisotropic temperature factors for the atoms in the stnrcturewere not successful.

The final atomic coordinates and the individual isotropic temperature factorswith the estimated standard deviations are given with the amount of displacementsof atoms from the ideal structure (Table 2). Fo and .F, values are on deposit.l

Dnscnrprrox oF THE Srnucrunn

The interatomic distances and the bond angles (Table 3 and 4)

were computed wiih the program RSDA-4 of the UNICS system(1967) based on the final atomic coordinates (Table 2). The estimated

standard deviations of the atomic coordinates were used to compute

the standard deviations in the interatomic distances and the bond

angles.

Deuiation from th"e id,eal structtae

The result of the refinement confirms the ideal structure proposed

by Bertaut (1953). Sulfur atoms approximately occupy the nodes of

the hexagonal closest packing and iron atoms are regularly arranged

1To obtain a copy of this material, order NAPS Document Number 01805.The present address is National Auxiliary Publications Service of the A.SJ.S., c/oCCM Information Corporation,866 Third Avenue, New York, New York 10022;and the price is $200 for microfiche or $5.00 for photocopies, payable in advance

to CCMIC-NAPS. Check a recent issue of the jqurnal for current address andprrce.

1072

TabLe 2. Final atonic cooldinates andplrrhot i te ( fe-S") obtained in this

TOKONAMI, N ISHIGACH I, AN D MORI MOT,O

Pyrrnotrte (Fe,so) obtained in thisi n p a r e n t h e s e s : " T h e c o l u n s o f a x l l , b * l A v l

isot lopic temperatule factors for atons lnstud.y. Est inated slandard deviat ions are

l A z lIacqent in Angstron froh-the-ideEl sE

l , a n d cruc ture by Bertaut (195

I rat

Fe3Fe4

s2

s4

0 . 3 8 0 9 ( 3 )0 . 3 8 1 5 ( 4 )0 . 3 5 8 7 ( 3 )0 . 3 7 5 00 . 2 0 6 1 ( s )0 . 5 4 1 8 ( 5 )0 . 2 0 9 6 ( 5 )0 . s 4 2 8 ( 5 )

0 . 0 7 00 . 0 7 90 . 1 9 4

0 , 0 2 50 . 0 0 L0 . 0 1 50 . 0 1 3

0 . 3 5 0 00 . 3 6 6 00 . 3 9 7 50 . 3 5 2 80 . 3 6 6 00 . 3 8 3 30 . 3 8 5 80 . 3 6 4 5

I I

1 )

l -5

O . I 7 I0 . 0 6 20 . 1 5 40 . r 5 20 . 0 6 20 . 0 5 70 . 0 7 40 . 0 7 2

- 0 . 0063o , r2340 r 2 5 0 I0 . 3 7 5 0

- 0 . 0 6 r 80 . 0 5 9 80 . I 8 0 50 . 3 0 8 ?

0 . 1 4 40 . 0 3 50 . 0 0 2

0 . 0 r 5

0 . r 6 00 . 0 8 7

0 . 9 4 ( r 00 . 8 9 ( B0 . 7 s ( 1 0I . 4 r ( 1 30 . 7 8 ( 1 50 . 6 r ( 1 40 . 5 0 ( r 40 . 6 5 ( 1 5

in the octahedral interstices of the sulfur atoms. The vacancies amongthe iron sites are in ordered arrangement and conflned to alternatelayers of iron atoms and normal to the c axis (Fig. 1). These vacanciesresult in slight displacements of iron and sulfur atoms from the posi-tions in the ideal structure proposed by Bertaut (Table 1). Theamounts of displacement of atoms in the final structure are given inAngstriim along the a,b, and c axes (Table 2).

Coordinations of iron and sullurr atoms

Because of the ordering of vacant sites for iron atoms and the dis-placements of iron and sulfur atoms from the ideal structure, coordi-nations of iron and sulfur atoms indicate some characteristics in thestructure under discussion in comparison with the structures of troilite(Evans, 1970) and of the 30-type pyrhotite (Fleet, 1921) .

All the iron atoms lie between six sulfur atoms with octahedralaruangements and also between other iron atoms, which are so closethat the iron atoms are coordinated both by sulfur atoms and by ironatoms as described later in detail.

The bond lengths and the bond angles around the iron atoms (Table3) indicate that the octahedra of sulfur atoms are not much distortedin this structure. The mean distances of Fe-S are 2.446, 2.M9,2.486,and 2.4M A for the octahedra of Fe( l ) , Fe(2) , Fe(3) , and Fe(4) ,respectively. These values a,re compared with the corresponding onesof 2.491 A in troilite (Evans, 1970) and 2.448 L in the BC-typepyrrhotite (Fleet, 1971).

All the Fe-Fe distances are more than 3.1 A in the basal iron layersexcept the Fe(3)-Fe(3) distance with 2.94+ A in the fi l led layer(Table 4). This makes a clear contrast with the structure of troilite,where the formation of triangular groups is characteristic in everybasal iron layer with the Fe-Fe distance of 2.glg A (Evans, lg70).However, the Fe-Fe distances between the neighboring iron layersare rather short along the c axis in the 4C-type structure, ranging from2.868 to 2.956 A (Table 3). They show systematic change along the c

1073

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t074 TOKANAMI, NISHIGUCHI, AND MORIMOTO

T3ble 4. Fe-Fe al istances in the vacancy containing and f i l , led i ron,layers. Est imated standald deviat ions are in parentheses, fhei r o n a t o m s a r e . l o c a t e d b y t h e f o l l o w i n g L e t t e r s t f , x = L / 8 ig , x = 3 / 8 i h , x = 5 / 8 , i , y e - L / 8 i j , y = ! . / e i k , y = 5 / 8 i e , y = j / 9 .

Vacancy-containinglayer

z > 3 / 8

Fi l led layer

Fe4 -Fe2hjFe4 -Fe2hkFe2hj -Fe2hk

3 . 2 6 3 ( 7 )3 . 6 1 3 ( 7 )3 . 4 3 4 ( 1 1 )

Fe3-FelhkFe 3 -Fe 19 t.

r e J - ! e f g a

3 . 6 7 8 ( 6 )3 . 1 0 9 ( 9 )3 . 2 0 1 ( 6 )3 . 7 6 0 ( 9 )

F a 1 - F a 1 F L

Felhk-Petg l ,F e l f j - F e l g i

9 2 6 ( 6 19 4 4 ( 6 14 r 8 ( 6 )s 1 4 ( 7 )

233

axis : 2.868 A for the central Fe (4) -Fe (3) distance increases to 2.956 Afor the Fe(2)-Fe(1) d is tance through 2.911 A for the Fe(3)-Fe(2)distance (FiS. 3). The linear arrangement of iron atoms along the caxis in the ideal NiAs structure (Table 1) can be compared withthe slightly bent arrangement in this structure where the anglesFe (1 ) -Fe (2 ) -Fe (3 ) , Fe (2 ) -Fe (3 ) -Fe (a ) , and Fe (3 ) -Fe (a ) -Fe (3) are 174.06", 166.05o, and 167.7lo,respectively.

Because a bonding interaction is expected between iron atoms withinteratomic distances of about 3.0 A or less (Goodenough, 1g62) , Fe(l)is coordinated by one iron atom [Fe(2)l Fe(2), and Fe(4) by twoiron atmos [Fe(2) by Fe(3) and Fe(4) , and Fe(a) bV two Fe(3) ] ,and Fe(3) by three i ron atoms fFe(2) , Fe(3) , and Fe (a)1, be-sides by the six sulfur atoms with octahedral arrangement describedabove (Figs. 3 and 4). The Fe-Fe bonding in this structure will bediscussed later in more detail.

The sulfur atoms are eoordinated by five iron atoms which are onthe corners of a trigonal prism because of a vacant site on one of thecorners. Only S(a) is coordinated by six iron atoms as in troilite(F is . 3) .

The structure as a whole can be described to be built up of chainelements of seven octahedra of sulfur atoms around iron atoms,elongating along the c axis, in which the octahedra are united bysharing two opposite faces. The chain elements thus formed are con-nected sideways by sharing edges with the neighboring ones (Fig. B).

Atomic arrangement around, the uacant site

Vacant sites:for iron atoms are orderly distributed in the vacancy-containing basal layers, and are surrounded by six sulfur atoms andtwo iron atdms. It is of interest to examine the arrangement of theseneighboring atoms around the vacant sites. The amount of displace-ments of these neighbors from the positions in the ideal structure are

MO N OC LIN IC PY RRH OT I I'E

z=#

z=H

z=i8

z=H

z - *

z , *

7 = 3- 1 6

z = *

7 = - 1- 1 6

z= - *

z - - *

Frc. 3. Schematic drawing of the crystal structure of the 4C-type pyrrhotite.

A group of seven octahedra of sulfur atoms around iron atoms builds a col-

lective unit along the c axis, in rvhich the octahedra are united by sharing two

opposite faces.

1075

z= 116

TOKONAXII. N ISHIGUC H I. AN D MORIMOTO

( 1 I tzt (31 t4tFrc. 4. The coordination schemes of iron atoms in the 4C-type pyrrhotite.

schematically shown in Figure 5, where the arrows represent the com-ponents of the displacements along the a, b, and c axes.

In order to compare the atomic arrangement around the vacant sitewith that around the iron atoms in the structure, a hypothetical site(X) was determined from the positions of octahedral sulfur atomsaround the vacant site so that all the X-S distances become approxi-mately same. The coordinates of the X site were obtained to be(0.3750, 0.3668, -0.1250). The mean values for the X-S, X-tr'e, andS-S distances around the X site are compared with those for the Fe-S,Fe-Fe, and S-S distances around the iron atoms (Table 5). The resultseems to indicate that the octahedra of sulfur atoms around the vacantsites are only slightly smaller in comparison with those around the ironatoms. The X-Fe distance is, however, significantly smaller than theFe-Fe distances.

Drscussrorq

Because the vacant sites are distributed in the structure as far aspossible from each other and the atoms around the vacant sites arelikely to morre to reduce the vacant space, the structure of the 4C-typepyrrhotite seems to be essentially ionic.

In order to obtain a clue to the distribution of Fe,* and Fe3*, thestabilization energies have been calculated for different distributionsof Fes* and Fe2* on the basis of the refined stmcture. The distributionsof Fe3* were limited into the following four typical models afterBertaut (1953),

MONOCLINIC PYRRHOTITE

model 1 (distribution of Fe' '"* at, all the iron sites),model 2 (distribution of Fe'* at the Fe(2) sites),model 3 (distribution of Fe'* at the Fe(l) sites) andmodel 4 (distribution of Fe'* at the Fe(4) sites and a part

of the Fe(l) sites).

The calculations were carried out by using a Madelung constantprogram (Unpublished report by Quintin Johnson of the LawrenceRadiation Laboratory, 1965) adapted to FACOM 230-60 computer atKyoto University. The stabilization energies are as follows:

L077

model 1model2model3model4

-276.3 kcal/mol,-395.6 kcal/mol,-409.9 kcal/mol and-408.5 kcal/mol.

The conclusion that the model 3 is most stable among the four modelsis different from that obtained by Bertaut (1953) based on his idealstructure, which gave the sta.bilization energies of -495 kcalmol forhis model 4 and of -472 kcal/mol for his model 3. Our result seemsreasonable because the electric unbalance caused by the vacancies of

iron atoms can be compensated by the additional charge of Fe(l)

atoms which are the nearest cations of the vacant sites.

T"'

aFrc. 5. Displacements of atoms around the vacant site from the positions in

the ideal structure prgposed by Berta,ut (1953). The a;ITows represent the com-ponents of displacements along the a, b, and c axes.

1078

a a D r e ) .

and

are

I'OKONAMI, NISHIGUCHI, AND MORIMOTO

The mean distances (A) for x-S, x-Fe and s-s around the vacant si te (x)those for Fe-s, Fe-Fe and s-S around iron atoms. Nuber in parenthesesthose of bonds considered.

arounal x around Fel around Fe2 around Fe3 around Fe4

x - s ( 6 ) 2 . 4 3 9s - s ( 1 2 ) 2 . 4 5 0s - s ( 3 ) 4 . 8 7 6x - F e ( 2 ) 2 . 7 0 8

s -s ( 12 )s - s ( 3 )

2 . 4 4 63 . 4 5 04 . 8 7 6

F e 2 - s ( 6 ) 2 . 4 4 9s - s ( 1 2 ) 3 . 4 5 8s - s ( 3 ) 4 . 8 8 9F e 2 - F e ( 2 ) 2 . 9 3 4

P e 3 - s ( 6 ) 2 . 4 5 6s - s ( r 2 ) 3 . 4 5 1s - s ( 3 ) 4 . 8 8 5F e 3 - F e ( 3 ) 2 . 9 0 8

F e 3 - s ( 6 ) 2 . 4 4 4s - s ( 1 2 ) 3 . 4 5 4s - s ( 3 ) 4 . 8 8 1F e 4 - F e ( 2 ) 2 . 8 5 8

However, the \{6ssbauer spectra of the C-type pyrrhotite show nosign of Fet* even at, 4.2oK (Hafner and Kalvius, 1966; Levinson andTreves, 1968). This means that even at this low temperature the Fe'*and Fet* ions do not have separate existence for a time longer than thatoccupied by the Mossbauer transition (10-?-10-8 sec.). These resultsseem to reveal that the stability of pyrrhotite should be treated on thebasis of an ionic model not of Feu'*Fert*Sr'-, but of Fer2 2s+S82- (model 1),although its calculated stabilization energy is not favorable.

Furthermore the observation of the Mijssbauer spectra of the4C-type pyrrhotite (Hafner and Kalvius, 1966; Levinson and Treves,1968) provides three different magnetic flelds at the sites of the ironatoms which are caused by the magnetic interactions between theiron neighbors in the structure. The application of an externalmagnetic field and the intensity ratio for the various subspectraindicate that the three different magnetic fields correspond to theFe(2) and Fe(4) , Fe(1) and Fe(3) (Levinson and Treves, 1968).Because the iron neighbors are different for these three kinds of ironatoms as explained above (Fig. 4) , the structure obtained in thisstudy explains well the results of the Miissbauer study.

According to Goodenough (1962), there are crit ical values ofcation-cation separation below which the d-electron must be treatedby means of collective system rather than localized one. In the Fe1-,Ssystem the crit ical separation is about 3.0 A, (Goodenough, 1962).Accepting this critical value for the Fe-Fe separation, the groupof seven iron atoms along the c axis ean be considered as a collectiveunit, in the 40-type pyrrhotite (Fig. 3) like the triangular group inthe troilite structure. Each iron group is connected with two othersthrough the short Fe(3)-Fe(3) bonds in the fi l led iron layer, formingan endless chain elongated along [101] (Fig. 6). The formation ofthese iron clusters possibly increases the stability of the apparentionic model of Ferz'zo+grz-.

The complicated structure of iron clustering found in the 4C-typepyrrhotite must be important for understanding of the long-rangeordering of vacant sites in the superstructures of pyrrhotite at various

- Fe3I

EezI

FeI

Fe I r Fe I r e3 i - r e3[ f l t l

F e 2 F e { I F e 2I i l t l

F e l ' F e 3 - F e 3 r F e li l [ t l

F e 2 ' F e { F e 2 'l l l t l

F e l F e 3 ' - F e 3 F e l r F e Ii [ [ [

Fe2 Fe4: Fe2 Ee2t l l r l

F e 3 - F e 3 r F e l F e I ' F e 3t l l l l l

Fe{ Ee2 ' Fe2 ' Fe4I I I I T

- Fe3 FeI rI

tre2I

P e l F e l '

tFe2 r

F e l F e 3 ' - F e 3t t t

Ee2 Fe4 ' Fe2l r l

Fe3 - Fe3 ' Fe It l

F e 4 F e z '

F e l ' F e ltl

Fe2tl

Fe I I Fe3I t l

F e 2 ' F e 4I t l

F e 3 r - F e 3i l t l

F e 4 ' F e 2I t l

- Fe3 I Fe Itl

Fe2 I

tFe I r Fe I

tlFez

tF e I ' F e 3

T TEe2' Fe4

MONOCLINIC PYRRHOTITE r079

F e 3 ' - F e 3 F e l !I r

Fe4 | Fe2i l I

- F e 3 ' F e l F e l r$ l

F e 2 ' F e 2 I

I rF e l ' F e I F e 3 ' -

I lEe2 Fe l '

I lF e I ' F e 3 - F e 3 '

t [ lFe2r Fe4 Ee2 '

T I IF e 3 r - F e 3 F e l '

t lF e 4 ' f e 2

l l- Fe3 ' Fe l Fe l l

t lF e 2 ' F e 2 r

a -

Frc. 6. Endless chains running parallel to t1011 composed of chain elements ofseven iron atoms elongated along the c axis.

temperatures and compositions (Morimoto, Nakazawa, Tokonami, and

Nishiguchi, 1971; Nakazawa and Morimoto, 1971).

ACKNoWLEDGMENTS

We express our thanks to Dr. Mabel Corlett for providing us with the speci-

mens from Kisbanya, Rumania, and to Dr. Quintin Johnson for providing us the

Madelung Constant Program. Thanks are also due to Drs. H. Nakazawa and

H. Horiuchi for illuminating discustions throughout this work, to Mr. A. Gyobufor drawing the figures, and to Miss M. Hirano for typing the manuscript.

Rr:rr:nnNcns

Benraut, E. F. (1953I Contribution ir, ll6tude des structures lacunaires' ,4cla

C r ystal logr.6, 537-56^.- (1956) Structure de FeS stoichiometriqne. Bull. Soc. Franc. Mhnral.

C rg stallo g r. 7 9, 276-292.BusrNc, w. R., K. O' MenuN,,rxn II. A. Lnvv (1962) ORFLS, A Fortran Crystal-

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Conr,nrr, Mennl, (1968) Low-iron polymorphs in the pyrrhotite group. Z. Kristal-Iogr. 126,124-134.

EveNs, H. T., Jn. (1970) Lunar troilite: Crystallography. Science,767,621423.Fr,nnr, M. E. (1971) The crystal structure of a pyrrhotite (Fe"S"). Acta Crgstal-

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nool

1080 TOKONAMI, N ISHIGACHI, A.N D MORIMOTO

Herxnn, SrnnaN, eNn Mrcrranr, ILqlvrus (1966)'The Mrissba,uer resonance of FeFin troilite (FeS) and pyrhotite (Feo*S). Z. Kristallogr. r2g, 44345&.

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Monruoro, N., H. Narezewe, K. Nrsnrcucur, eNl M. Toroxerrr (1970) Pyrrho-tites: Stoichiometric compounds with composition Fe-,S" (z ) 8). Science,168. 964-966.

-, -t M. ToroNennr, ern K. Nrsnrcucrrr (1971) Pyrrhotites: Structuretype and composition. Soc. Mining Geol. Ja,p., Spec. Issue, 2, 15-21. [Proc.IMA-IAGOD Meeting'70. Joint-Symp. Vol.J

NerAzawe, E[., aNn N. Monrrroro (1g71) Phase relations and zuperstructures ofpyrrhotite, Fer-,S. Mater. Res. BuIl.6, B4b-358.

Per,ecno, C., H. BnnlreN, eNn C. Fnorvnnr, (1g55) Dana's Sgstem ol M,i,neralogg,Vol. t. John Wiley and Sons, New York, p. 2BB.

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Mantncript receiuecl, February 11, 1972; accepted, lor publication, Febntnry pS,1972.