American Mineralogist, Volume 81, pages I 19-12 5, 1996

Structural relationship between pyrite and marcasite
IsrvAr.r D6oorwrr MrnAr.v P6srnr2'* lNn Pernn R. Busncx2
lDepartment of Mineralogy, Eiitvtrs Lor6nd University, Budapest,Mrizeum l$1.4/4, H-1088 Hungary'?Departmentsof Geology and Chemistry, Arizona State University, Box 871404, Tempe, Arizona 85287-1404,U.S.A.

ABSTRAcT
We studied pyrite spherutesbuilt of radiating crystals to establish the relationships
among microstructure, composition, and macroscopic appearance.Results of high-resolution transmission electron microscopy (HRTEM) imaging and selected-areaelectron
diffraction (SAED) show that many pyrite crystals contain planar faults perpendicular to
one of the [001] axes.A comparison betweenHRTEM micrographsand imagessimulated
for defect model structuresindicatesthat the faults can be interpreted as single( 10I ) layersof marcasite that disrupt the regular sequenceof (002) layers in pyrite. Such a marcasite
layer can be describedas a boundary between two pyrite crystals that are related by a 2,
screw axis parallel to [00]. Energy-dispersiveX-ray spectrometry (EDS) indicates thatsuch disordered pyrite crystals contain about 3 atoloAs. However, the distribution of As
is uniform over heavily faulted and fault-free regions, indicating the As content is not
related to the occurrenceof marcasitelamellae in pvrite.

IxrnooucrroN

found by Fleet (1970)in a study of the inversionof marPyrite is the most abundant sulfide mineral; it is wide- casite to pyrite.
TEM studiesby Van Goethem et al. (1978), Fagot etspread in ore deposits and common in many sedimentary, metamorphic, and igneousrocks. It exhibits a range al. (1978),and Fleet et al. (1989) showedthat planar deof interesting structural characteristicssuch as penetra- fects occur in pyrite parallel to {100}. The faults weretion twins, epitaxial inter- and overgrowthswith its poly- interpretedby Van Goethem et al. (1978) as antiphasemorph marcasite,and the occurrenceof a variety of mor- boundarieswith a displacementvector 7z[110].Fagot et
phologicaltypes(Tokody 1952;Donnay et al. I 977; Bonev al. (1978) characterizedthe defectsas stackingfaults with
R : +0.29[010].However,Fayardet al. (1980)showed
et al. 1985).
polymorphs.
Pyrite and marcasiteare
Fe atoms are oc- that the fault vector does not lie in the plane ofthe defect
They also mentahedrally coordinated in both structures.The octahedra and identifiedit as R: a10,0.27,0.51.
share corners in pyrite, whereasin marcasite they share tioned that such faults produce a marcasite lamella inedges(Buerger1931).Pyrite is cubic (a : 5.428 A; wittr pyrite. A similar interpretation was given by Fleet et al.
spacegroup Pa3 (Bragg I 9 I 3; Finklea et al. 1976), where- (1989) for the faults that they observed in pyrite. We
found that none of these models correctly describesthe
as marcasiteis orthorhombic (a : 4.443, b : 5.424,c :
3.387 A) with spacegroupPnnm (Buerger1931;Brosti- marcasite-pyrite structural relationship; the defect strucgen et al. 1973;Tossellet al. 1981).A characteristicfea- ture producedby the R : a[0,0.27,0.5]displacementvecture of both minerals is that S atoms form covalently tor (Fayard et al. 1980) producesa structural unit that issimilar but not identical to marcasite.
bonded pairs.
The goal of this study is to describe the geometry of
The structural relationship between pyrite and marcathe
defectsthat we observedin disorderedpyrite crystals.
site has been discussedin several studies. Brostigen andWeinterpret the faults on the basis of a new model for(1970)
proposed
Kjekshusa hypotheticaltransformationof pyrite into marcasite through the reorientation of the the marcasite-pyrite structural relationship and discuss
S-Spairs. They noted that the pyrite {001} and the mar- the results of EDS analvsesof faulted crvstals.
casite{l0l} planesshow the same atomic arrangement.
When epitaxial inter- and overgrowths of marcasite andExpnnrunNTAL METHoDS
pyrite occur, the two crystals commonly join in such a
We studied pyrite samples from the Recsk porphyry
way that {00 1}o".* is parallel to ( 10 1)."*.,," with ( 100 )pr"copper ore deposit in Hungary. The ores occur in an Eoparallel to [010].-*.,* (Brock and Slater 1978; Gait and cene subvolcanic andesite body that intruded Triassic
Dumka 1986). The same orientation relationship was carbonate rocks. Contact-metamorphic processesled to
the formation of a 150-250 m thick mantlelike skarn* Present address:Department of Mineralogy, University ofzone around the intrusion (Morvai 1982). Pyrite formed
Veszpr6m,P.O. Box 158, Veszpr6mH-8201, Hungary.
during severalstagesofore deposition; the hydrothermal/ 0r024r19$05.00
0003-o04x/96

119

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DODONY ET AL.: MARCASITE.PYRITE STRUCTURAL RELATIONSHIP

a,

a2
(101)

Frcunr 2, Modelfor a faultedplrite structure.The fault plane
is markedby the dashedline ([l00] projection).The diamonds
mark the positionsof the 2, axesthat are parallelto [100].R
vector(R : 0.115[010]).
displacement
denotes
theprojectedcrographswere simulated using Cerius software on a Silicon Graphics workstation; we processedand filtered digitized images using Gatan Digital Micrograph 2.1 on a
Macintosh computer. Analytical electron microscopy was
performed at 200 kV using a JEOL 2000FX electron microscope.We acquired EDS spectra with an ultra-thinwindow Kevex X-ray detector and then processedandprojected
along[100]; quantified them using the Quantex program. K factors
ofpyrite,
Frcrnn 1, (a) The structure(b) the structureof marcasite,projectedalong[010].Fe atoms for Fe and As were experimentally determined from AsThe whiteregionsin free pyrite grains and arsenopyrite (FeAsS),respectively.
S atomsarelight spheres.
aredarkspheres,(lowerleft corners)arecenthe simulatedelectronmicrographs
Thewhitelinesdefineunit cells.
Srnucrunn MoDEL
teredat the structuralchannels.
In the pyrite structure the S-Sdoublets are aligned parsamples that we studied occur in lenseswithin the car- allel to the body diagonalsofthe cubic cell. Ifthe strucbonate rocks, a few meters outward from the skarn zone. ture is viewed along [00], the projectedpositionsof the
The acicular,2-4 mm long radiating crystalsform small Fe atoms are at the centersof the S-Sbonds (Fig. la). For
spherules. Pieces of the spherules were crushed under this viewing direction the S-Fe-Striplets are alternately
chloroform, and drops of the suspensionwere allowed to oriented in projected NE-SW and NW-SE directions
dry on holey carbon TEM grids. Ion-milled specimens along [001], whereasin rows parallel to [010] they all lie
in either the SW-NE or the NW-SE direction.
were also prepared.
The similarities and differencesbetween the structures
HRTEM imageswere obtained at 400 kV with a JEOL
4000EX transmissionelectron microscopeequippedwith of pyrite and marcasite are best seen by comparing thea double-tilt goniometer stage(x,y : +25).Electron mi- [100] projection of pyrite with the [010] projection of mar-

DODONY ET AL.: MARCASITE-PYRITE STRUCTURAL RELATIONSHIP

casite(Fig. lb). The two polymorphs sharethe samebasic
structural unit. A single S-Fe-Slayer parallel to (l0l) in
marcasiteis identical to an S-Fe-Slayer parallel to (00/)
in pyrite. However, thesebasiclayersare arrangedaccording to a different schemein the two structures.Instead of
forming a zrgzagpattern as in pyrite, all S-S doublets areparallel to one another in this projection of marcasite.
The computed HRTEM images in the lower left corners of Figures la and lb were obtained at Scherzerdefocus for 54 A thick crystals.At theseconditions the contrast in the micrographsis directly relatedto the structure.
The white regions correspond to the channels between
S-Fe-S triplets. The arrangementsof the white spots inthe two imagesreflect the differencebetweenthe stackingof S-Fe-Slayers in pyrite and marcasite.
The pyrite-marcasite structural relationship can be described in terms of a pyrite structure that contains a planar fault (Fig. 2). In [100] projection, S-S pairs are oriented on both sides of the fault plane parallel to the twobody diagonalsin the (0ll) plane ofpyrite, instead oflying alternatelyin the (0T11and (01l) planes.The centers of the S-S pairs (the Fe atoms) are shifted by 0.115[010]. Such an arrangement occurs at the boundary of
two pyrite crystals (I and II) that are brought into coincidenceby a 2, screw axis parallel to I I 00], the direction
ofprojection. The pair ofS-Fe-S layerson the two sidesof the boundary form a single (l0l) layer of marcasite(most easily seen when Fig. lb is rotated clockwise by
90").
Depending on its position relative to the pyrite unit
cell, the 2, axis produceseither a (l0l) or a (l0l) Iayero f m a r c a s i t eI f. t h e 2 , a x i so c c u r sa t y : 0 . 1 3 5 , z : 0 . 2 5(as marked in Fig. 2), the boundary structure is identicalto a marcasite(l0T) layer; if it is at | : -0.135, z :-0.25, then a (l0l) Iayer of marcasiteforms.

Rrsur-rs

Frcunp3. HRTEM imageof a pyrite crystal([l l0] projection) containingseveralsingleplanarfaults(markedby arrows).
markedL and R (seetext).
Faultsoccurwith two orientations,

Many pyrite crystals contain faults perpendicular to one
ofthe (001) axes.The faults occur in bunchesand resultin a recognizablecontrast changein HRTEM imagesobtained from (100)- and (1l0)-type directions (Fie. 3). 4A,. The only periodic pattern that can be observed apThe SAED pattern reveals that the faults are parallel to pearsat the fault. The FT ofthe boxed area in Figure 4C(001):There is faint continuousscatteringparallel to [004* confirms that the diffuse intensity maxima between the
between reflections belongrng to ordered pyrite. In the pyrite reflectionsarise from the faulted region. [A discushigh-resolution image the stacking of (002) layers is sion, with examples, of the manipulation of Fourierchangedby the defects,and two types offaults are distin- transformsis given by Buseck(1992, Figs. l-6).1
When the pyrite reflections and the diffuse scattering
guished. In fault-free regions of the crystal every second
positioned;
faultstheto the image but all the background intensity
identicallycontributeiswhite spot along [001]
introduce left (marked L) and right (marked R) sequences is filtered out (Fig. 4D), both the ordered pyrite regions
and the faults appear with enhanced contrast. We prointo the ordered stacking of(002) layers.
Figure 4A is an unprocessed,digitized micrograph, in duced Figure 4E by subtracting 4D from 4A,. Only back[100] projection,of a plrite crystalthat containstwo faults. ground noise appears, showing that all valuable inforThe Fourier transform (FT) of the image shows diffuse mation in the FT is contained within the reciprocal latticeintensity along the reciprocal lattice rows that are parallel rows that are parallel to [00/]*.
The pyrite crystal in Figure 5 contains both completely
to [004*. If the diffuse intensity in the FT is masked andonly the pyrite reflections are used to construct Figure ordered and heavily faulted regions.In the SAED pattern
48, the contrast ofthe faulted region changesrelative to there are diffuse spotsbetweenthe pyrite reflectionsalong
Figure 4A. Figure 4C is produced by subtracting4B from the reciprocal lattice rows parallel to [00/]*, indicating

122

DODONY ET AL.: MARCASITE.PYRITE STRUCTURAL RELATIONSHIP

Frcunp 4. (A, top and bottom) Unprocessed,digitized imagesof a faulted crystal and its computed Fourier transform (inset).
The faults are marked by arrows. (B) Filtered image of the area in A, produced by only the pyrite Bragg reflections.(C) The A B subtraction results in an image that shows the nonpyrite part ofthe crystal. The inserted Fourier transform is calculatedfor theboxed area. (D) Reconstructedimage produced by both pyrite and nonpyrite reflections.(E) The result of the A - D subtraction.

that the faults are oriented parallel to (00/) and perpendicular to those S-Fe-S layers in which the S-S bonds
form a zigzag paltern. Therefore, the orientation of thefaults is the same as in the structure model discussed
above.
The high-resolution imagesshown in Figure 6 were obtained from the region marked AB in Figure 5. These
structure images were interpreted on the basis of the pyrite-marcasitestructuralrelationshipdescribedabove.The
model shown in the insert of Figure 6b was used to produce simulated images. We obtained a good match between calculatedand experimental images(as seenin thelower right corner of Fig. 6a) and confirmed that thestructural relationship betweensomeadjacentS-Fe-Slayers is the same as that between layers of marcasite (M),whereasother layer pairs are oriented as in pyrite (P) (Fig.
6b). As determinedfrom high-resolutionimages,the M:P
ratio is about 1:2 in the heavily faulted region AC of
Figure 5.
The alternation of pyrite and marcasitestructural units
producesquasi-periodicsequencesof S-Fe-Slayers. At
the bottom ofFigures 6aand 6b thereis a ten-layerband

of marcasite. Elsewhere, in the disordered part of thecrystal, both (101) and (l0l) layers of marcasiteoccur.
Various layer sequencescan be observed, with the fourlayered PPPM unit appearing the most regularly. The
presenceof this variety of structural units gives rise todiffuse diffraction maxima along the [}kt]* (k : 2n) reciprocal lattice rows parallel to c* (Fig. 5). The nonpyrite
diffraction spots divide the spacingsbetween pyrite reflections by nonintegral values.
The pyrite spherulescontain up to 4.5 atoloAs. Because
the structure of arsenopyrite is similar to that of marcasite, we hypothesized that As is contained in the structural defects or, at least, that the defects are associatedwith elevated As content.
We analyzedthe crystal shown in Figure 5 using EDS.
X-ray spectrawere collected from the heavily faulted regron AC and from areasof ordered pyrite on either sideof AC. Theseanalysesindicate that the distribution of As
is uniform over the ordered and disordered regions. The
crystal contains -2.7 ato/oAs, and the variation of the Asconcentrationremains within the analyticalerror (+0.3
ato/o\.

DODO}IY ET AL. : MARCASITE-PYRITE STRUCTURAL RELATIONSHIP

t23

DrscussroN
The formation of structural defects in pyrite can beinterpreted as either a growth phenomenon or a result ofa marcasite + pyrite transformation. According to Murowchick (1992), pyrite formed by the inversion of marcasitecan be recognizedin polished section;it is anisotropic, contains numerous pores that form characteristic
patterns, and in many places it consists of bundles ofcrystals in two distinct orientations relative to the marcasite parent. The structurally disordered pyrite from
Recskis optically anisotropic; however, neither pores nor
two distinct pyrite orientations were observed.Therefore,
acceptingMurowchick's (1992)results,we concludethat
the pyrite spherulesare primary products of hydrothermal deposition and interpret the marcasite lamellae as
growth defects.
The controlling mechanism of fault formation is unknown. Marcasite forms from acidic aqueous solutions(pH < 5) at low temperature(T < 240'C), with HrS!
presentat the site of deposition (Murowchick and Barnes
1986;Schoonenand Barnes199Ia, l99lb). It is possiblethat at the time of nucleation conditions episodically favored the formation of marcasite.Perhapsthis is why thefaults tend to concentrate into groups between orderedregionsofpyrite.
We studied the relationship betweenmacroscopicmorphology and atomic structure by looking at ion-milled
samplesthat were prepared to provide views from boththe ends and the sidesof pyrite fibers. Marcasite lamellae
are abundant in images obtained from end-on views,
whereasthey seem much less common in images takenfrom directions perpendicularto the elongation directionof the crystals. It appears that the marcasite layers areoriented parallel to the directions in which the pyrite
crystalsgrew most rapidly (along[100] of eachcrystal).
In many placesthe marcasitelamellaeterminate at grainboundaries. Owing to the slight mismatch between thedimensionsof the pyrite (002)and marcasite(l0l) layers(0.130/o
between[00]o".i,"and [010]*"*"",,"and 1.25o/obetween [010]ov.eord [01]-..*"t," dimensions),strain canaccumulate along a marcasite lamella. This strain is released by the formation of grain boundaries along thetermination of sets of defects. In this way, the presenceof a marcasite lamella in pyrite limits the growh of thecrystalalongthe marcasite[101] axis,which is the direction of the larger mismatch between the two structuralunits. Hence, it is possible that the planar defectscontrol
the cross-sectionalsizeofpyrite fibers and are responsible
for the characteristicmorphology of the pyrite spherules.

Frcunr 5, Low-magnification,[100] electronmicrograph and
SAED pattern ofa pyrite crystal that contains both disordered
and ordered regions (seetext).

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DODOI.ry ET AL.: MARCASITE-PYRITE STRUCTURAL RELATIONSHIP

P
P
M
P
P
P
M
P
P
P
M
P
M
P
P
P
M
P
P
P
M
P
P
P
M
P
M
P
P
P
M
P
P
M
P
M
M
M
M
M
M
M
M
M
P
P
Ftcunr 6. (a) Comparison between a simulated micrograph(lower right corner) and the experimental high-resolution image
obtained from region AB of the crystal in Figure 5. The simulated micrograph was calculatedfor a 54 A thick crystal at -500
A defocusand 0.9 mrad beam tilt. (There is a slight mismatch
betweenthe experimentaland simulated micrographs,producedby the processof model building. The model was constructedby
cutting and pasting fragments of a pyrite structure; therefore, itcannot reproducethe small differencesthat exist betweenthe correspondingdimensions in pyrite and marcasiteslabs.)(b) Comparison between the structure model and the experimental image
for the bottom part of region AB. M : marcasite,p : pyrite.

DODONY ET AL.: MARCASITE-PYRITE STRUCTURAL RELATIONSHIPAcKNoWLEDGMENTS
We thank John Rakovan for calling our attention to some studies on
the morphology of pyrite. Discussionswith David Smith on the effectsof
beam misalignment in HRTEM are gratefully acknowledged.This study
was supportedby National ScienceFoundation (NSF) grant EAR-9219376(to P.R B.). We used TEMS located in the Center for High Resolution
Electron Microscopy, which is supportedby the National ScienceFoundation under grant no. DMR-861 1609 I.D. was supportedby Hungarian
National ResearchFoundation (OTKA) gxantT007545. We thank Bruce
Hyde for his helpful comments.

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