B. LESKOVAR et al.: TEMPERATURE-INITIATED STRUCTURAL CHANGES IN FeS2 PYRITE ...
259–265
TEMPERATURE-INITIATED STRUCTURAL CHANGES IN FeS2
PYRITE FROM POHORJE, EASTERN ALPS, NORTH-EASTERN
SLOVENIA
S TEMPERATURO POVZRO^ENE STRUKTURNE SPREMEMBE
FeS2 PIRITA IZ POHORJA, VZHODNE ALPE, SEVEROVZHODNA
SLOVENIJA
1Bla` Leskovar, 2Mirijam Vrabec, 2Matej Dolenec, 1Iztok Nagli~, 2Tadej Dolenec,
3Evgen Dervari~, 1Bo{tjan Markoli
1University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, Ljubljana, Slovenia
2University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Ljubljana, Slovenia
3University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Mining and Geotechnology, Ljubljana, Slovenia
leskovarblaz@gmail.com
Prejem rokopisa – received: 2015-11-02; sprejem za objavo – accepted for publication: 2016-03-31
doi:10.17222/mit.2015.328
X-ray phase analysis (XRD), differential thermal analysis (DTA) and X-ray fluorescence spectroscopy (XRF) of pyrite samples
in a protective (Ar) and an oxidative atmosphere (air) at different temperatures (200, 300, 500, 700) °C and times (1, 2, 4) h
were made. A graphical representation of the unit cell of magnetic pyrrhotite and diffraction patterns of the individual crystal
structures of pyrite, which occur in the sample at selected temperatures, atmosphere, and exposure times were analyzed with
X-ray diffraction software (CrystalMaker). The results show that FeS2 exhibited temperature-dependent changes in the crystal
structure that are not included in current Fe-S phase diagrams.
Keywords: Pohorje, Eastern Alps, structural changes, FeS2 pyrite, XRD, XRF, DTA
Vzorci pirita so bili analizirani s pomo~jo rentgenske fazne analize (XRD), diferen~ne termi~ne analize (DTA) in rentgenske
fluorescentne spektroskopije (XRF) po `arjenju v za{~itni (Ar) in oksidativni atmosferi (zrak) pri razli~nih temperaturah (200,
300, 500 in 700) °C in ~asih (1, 2 in 4) h. S programsko opremo (CrystalMaker) je grafi~no predstavljena osnovna celica
magnetnega pirotita. Analizirani so tudi rentgenogrami posameznih kristalnih struktur pirita, ki se pojavijo v vzorcu pri
dolo~enih temperaturah, atmosferi in ~asih `arjenja. Rezultati ka`ejo, da pirit izkazuje temperaturno odvisne spremembe v
kristalni strukturi, ki niso vklju~ene v dosedanje fazne diagrame Fe-S.
Klju~ne besede: Pohorje, Vzhodne Alpe, strukturne spremembe, pirit (FeS2), XRD, XRF, DTA
1 INTRODUCTION
Pyrite is the most common and widespread of the
sulfide minerals, found in a wide variety of geological
formations. It occurs as magmatic segregations, as an
accessory mineral in igneous rocks, in contact meta-
morphic deposits, in large hydrothermal veins and in
many sedimentary rocks. In these rocks the pyrite is
usually found associated with other sulfides or oxides
and is often mined for the gold or copper associated with
it. Despite a high Fe content (46.55 % of mass fractions),
pyrite has never been used as a significant source of iron.
Because of the large amount of sulfur present in the
mineral, it is used as an iron ore only in those countries
where oxide iron ores are not available. Iron is produced
from pyrite by roasting, causing complete oxidation and
removal of the sulfur; the latter is the reason for the
brittleness in iron and its alloys.1,2 During the early years
of the 20th century, pyrite was used as a mineral detector
in radio receivers, and is still used by šcrystal radio’
hobbyists. Pyrite detectors occupied a midway point
between galena detectors and the more mechanically
complicated perikon mineral pairs.3,4 Pyrite is a
semiconducting material with band gap of 0.95 eV.5 It
has been proposed as an abundant, inexpensive material
in low-cost photovoltaic solar panels.6 Pyrite still
remains in commercial use for the production of sulfur
dioxide, in the paper industry, and in the production of
sulfuric acid. It is also a source of sulfur, for the
production of tires, explosives, disinfectants, medicines,
ink, wood preservatives, dyes and matches.1 Pyrite is
also used as the cathode material in the Energizer brand
of non-rechargeable lithium batteries.7
Although pyrite is the principal constituent of many
ore bodies, it is absent in some high-temperature depo-
sits formed from liquids and gases, suggesting clear
dependency of the pyrite crystallization under restricted
temperature conditions.8 In this paper, the phase trans-
formation of pyrite as a function of temperature, time
and atmosphere are studied in order to clarify its stability
and changes in crystal structure in the range of 33 % of
mass fractions to 60 % of mass fractions of S in an inert
atmosphere and the path of pyrite decomposition in the
oxidative atmosphere.
Materiali in tehnologije / Materials and technology 51 (2017) 2, 259–265 259
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 66.017:549.324/.326(234.32)(497.4) ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)259(2017)
2 GEOLOGICAL BACKGROUND AND SAMPLE
LOCATIONS
The Pohorje Mountains are located at the south-
eastern margin of the Eastern Alps in north-eastern Slo-
venia. They represent ultrahigh-pressure metamorphic
terrane and are built up of three Eo-Alpine nappes that
belong to pre-Neogene metamorphic sequences of
Austroalpine units of the Eastern Alps. Structurally, the
lowest nappe represents the Lower Central Austroalpine
and consists of medium- to high-grade metamorphic
rocks, predominantly micaschists, gneisses and amphi-
bolites with marble and quartzite lenses. It also contains
several eclogite lenses and a body of metaultrabasic
rocks. The Pohorje nappe is overlain by nappe composed
of weakly metamorphosed Paleozoic rocks, mainly low-
grade metamorphic slates and phyllites. The uppermost
nappe is built up of Permo-Triasic clastic sedimentary
rocks, mainly sandstones and conglomerates. The two
latter nappes represent the Upper Central Austroal-
pine.9,10 The entire nappe stack is overlain by Early
Miocene sediments that belong to the svn-rift basin fill
of the Pannonian Basin.11 The central part of Pohorje is
occupied by granodioritic to tonalitic intrusion of
Miocene age (18-19 Ma).12,13
Pyrite mines in Zgornja Polskava are located along
the creek where several exploratory tunnels were dug in
a sequence of metapelitic rocks. The GPS geographic
coordinates of the mines are as follows: N 46°25’54.8’’,
E 15°35’40.0’’.
Pyrite crystals occur within tremolite forming more
than 50-cm-thick layers or veins in metapelitic country
rocks.14,15 These are mostly gneisses and micaschists that
formed under high-pressure and high-temperature condi-
tions of 2.2–2.7 GPa and 700–800 °C. They are medium-
grained rocks with a granoblastic texture composed
mostly of quartz, mica, biotite, garnet, kvanite, plagio-
clase and K feldspar. Metapelitic rocks in the investi-
gated area are more or less intensively limonitized.9,10,16
3 FORMATION OF PYRITE AND SAMPLE
DESCRIPTION
The samples of pyrite that were used for the analyses
were taken from the mine in Zgornja Polskava, which
was operational from 1916 to 1920. The ore was mainly
used for the production of sulfuric acid. In abandoned
mine tunnels, pyrite cubes from 2 mm to 20 mm in size
and more can still be found today.15,17 From Figure 1 it is
clear that analyzed samples are brass yellow modified
cubes with typical pyrite striations on the surface. In
places pyrite crystals are limonitized and contain rare
quartz veins. The size of the samples is approximately
(20 × 20 × 20) mm.
Pyrite forms in high-temperature veins at tempera-
tures from 300 °C to 400 °C.16 The rock around the veins
is characterized by a red color, which is due to the
presence of hematite dispersed in the sequence of
metapelitic rocks. For hydrothermal ore deposits of this
type it is characteristic that sulfides are found in the
green, gray and black layers, which alternate with the red
layer.18
The binary phase diagram Fe-S reported by Kuba-
schewski19 is shown in Figure 2. The crystal structure of
pyrite (, ) transforms into the pyrrhotite (, , ) when
the sulfur concentration is decreasing or the temperature
is rising. With increasing temperature the pyrite decom-
poses into pyrrhotite (FeS) and liquid at a temperature
of 743 °C. Because of the low boiling point of sulfur
(444.6 °C) at atmospheric pressure the free bonded
atoms evaporate (irreversible reaction). With decreasing
temperature the crystal structures of FeS (P63/mmc)
transform to FeS (P62c) at 315 °C and then FeS into
FeS (unknown crystal structure by Kubaschewski).
4 MATERIALS AND METHODS
4.1 X-ray powder diffraction (XRD)
A Philips X-ray diffractometer with a PW3830 gene-
rator was used for the XRD analyses. The patterns were
recorded with the following parameters: voltage 40 kV,
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Figure 2: Phase diagram Fe-S19
Slika 2: Fazni diagram Fe-S19
Figure 1: Analyzed samples of pyrite
Slika 1: Vzorci pirita uporabljeni za analize
current 30 mA, wavelength of light used X-ray Cu–K
0.15418 nm, secondary graphite monochromator and
proportional counter. The angle 2 was recorded in the
range of 0° to 70°, at a rate of 1.2° 2/min. The limit of
detection of the minerals in the sample is 1–3 %.
4.2 X-ray fluorescent spectroscopy (XRF)
A portable (field) X-ray fluorescence analyzer
NITON (model XL3t + GOLDD-900S He) was used to
determine the quantity of each element in the sample.
The method allows a quantitative analysis of more than
80 elements, from magnesium to uranium. When
measuring, we used the module "Mining". During the
measurement the sampling site was pumped with helium
gas for the better detection of light elements (Mg, Si, Al,
P). The measurement time for each sample was 210 s.
4.3 Differential thermal analysis (DTA) and differen-
tial scanning calorimetry (DSC)
DSC was performed on a STA 449 Jupiter device by
NETZSCH. All the analyses were carried out in a
protective argon atmosphere or air. The experiments
were conducted at heating/cooling rates of 10 °C/min,
according to the Heat Flux mode. In this mode the
investigative and comparative sample are heated with the
same heat source.
5 RESULTS AND DISCUSSION
The phase transformations of pyrite are defined as a
function of temperature, time and atmosphere. The
crystal structures of the phases that occur in the sample
at various temperatures and times in the presence of a
protective (Ar) or the oxidizing atmosphere (air) were
determined by means of X-ray structural analysis. The
purity of each sample was identified using X-ray fluore-
scence spectroscopy. The presence of the chemical
reactions or changes in the crystal structure was traced
by DSC. The temperatures of the heating were (300, 500
and 700) °C. At each temperature we held the samples
for (1, 2 and 4) h. The DSC experiments were carried out
according to a temperature program of 10 °C min–1 to
800 °C to trace changes in FeS2 that were not found in
Fe-S phase diagram. The crystal structure of the sample
was defined with the help of a computer program
CrystalMaker and Atlas of Crystal Structure Types for
Intermetallic Phases.20,21
5.1 Annealing pyrite in air at 300 °C
In the crystal structure of pyrite no changes occurred
after annealing at a temperature of 300 °C for 4 h. The
sample consisted of pyrite and quartz. The diffraction
pattern for the sample after annealing at 300 °C for 4 h is
shown in Figure 3.
The unit cell of pyrite is a cubic crystal lattice with a
point symmetry by Hermann-Mauguin notation (HM)
m3 (there is mirror plane in the direction of a and 3-fold
rotational axis with inversion (3) in the direction of b
axis), and a space group Pa3, that illustrates the primitive
lattices (P), a glide plane in the direction of the a axis
and the 3-fold rotational axis with the inversion (3) in the
direction of the b axis. The unit cell of -quartz repre-
sents a trigonal crystal system with a point symmetry
(HM) 32 (which illustrates the 3-fold axis in the direc-
tion of the a axis and 2-fold rotational axis in the direc-
tion of the b axis) and the space group P3221 (primitive
lattice (P), the screw axis with the 3-fold rotation in the
direction of a axis (triad) and a 2-fold rotational axis in
the direction of the b axis).
5.2 Annealing pyrite in air at 500 °C
Already after one hour of annealing at 500 °C a
change in the crystal structure appeared. It can be seen
that the intensity of the peaks representing the crystal
structure of pyrite decreased (the amount of the crystal-
line structure of pyrite decreased) and new peaks arose,
which represented new crystal structures. With the help
of diffraction patterns it was found that along with the
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Figure 4: XRD analysis of a sample annealed at 500 °C for 4 h
Slika 4: Rentgenogram vzorca `arjenega 4 h na temperaturi 500 °C
Figure 3: XRD analysis of a sample annealed at 300 °C for 4 h
Slika 3: Rentgenogram vzorca `arjenega 4 h na temperaturi 300 °C
crystal structure of pyrite and the quartz crystal structure
of hematite (Fe2O3) and mikasaite or iron(III)sulfate
(Fe2(SO4)3) appeared. The amount of hematite increased
with the annealing time, while the amount of pyrite and
iron(III)sulfate decreased. The amount of quartz was
equal during the time of annealing. Figure 4 has the
diffraction pattern of a sample made after annealing at
500 °C for 4 h.
The unit cell of the iron(III)sulfate belongs to the
trigonal crystal system with a point symmetry (HM) 3
(3-fold rotation axis with inversion in the direction of a
axis), and a space group R3 (rhombohedral lattice R and
the 3-fold rotation with the inversion in the direction of
the a axis). The basic cell of hematite (-hematite)
belongs to the trigonal crystal system with a symmetry
point (HM) 32 /m (which represents a 3-fold rotational
axis with the inversion in the direction of the a axis and a
2-fold rotational axis with the mirror plane in the direc-
tion of the b axis) and space group R c3 (rhombohedral
lattice R, 3-fold rotation axis with inversion in the direc-
tion of the a axis, and a glide plane in the direction of the
c axis).
5.3 Annealing pyrite in air at 700 °C
Already after one hour of annealing at 700 °C the
sample consisted of the crystal structure of hematite with
only a small amount of quartz. The temperature was so
high that the sulfur evaporated and reacted with the oxy-
gen in the atmosphere to form sulfur dioxide. Oxygen
from the atmosphere diffused into the sample and
formed hematite. Figure 5 shows the diffraction pattern
of the sample made after annealing at 700 °C for 4 h.
5.4 The X-ray fluorescent spectroscopy
Table 1 presents the weight percentages of the che-
mical elements and the relative error in the calculation of
weight fraction of the chosen elements. Presented are the
three samples that were heated to 300 °C, 500 °C and
700 °C and exposed for 4 h to air.
The sample annealed at 300 °C had about 37 % of
mass fractions of Fe, and approximately 60 % of mass
fractions of S. Due to the fact that the chemical com-
position of pyrite corresponds to 46.55 wt. % Fe and
53.45 % of mass fractions of S, it can be concluded that
some atoms of sulfur are free and bonded to the surface
of the sample. It can also be assumed that a small
amount of tremolite Ca2Mg5Si8O22(OH)2 is present (in
which one can find pyrite) and some metamorphic rocks,
specifically gneiss (location of exploratory tunnels of
pyrite), due to the presence of other chemical elements.
Table 1: Chemical composition of samples annealed at (300, 500 and
700) °C in air (X-ray fluorescent spectroscopy, w/%)
Tabela 1: Kemi~na sestava vzorcev `arjenih na zraku pri temperaturah
(300, 500 in 700) °C (rentgenska fluorescentna spektroskopija, w/%)
Tempe-
rature
Chemical element
Fe Fe Error S S Error Si Si Error
300 °C 37.49 0.14 60.77 0.13 0.01 0.02
500 °C 53.72 0.46 17.63 0.11 4.37 0.05
700 °C 71.82 0.75 0.18 0.01 6.65 0.07
Al Al Error Ca Ca Error Cl Cl Error
300 °C 0.04 0.07 0.02 0.01 0.97 0.02
500 °C 2.98 0.11 0.13 0.02 0.14 0.01
700 °C 1.03 0.06 0.19 0.02 0.23 0.01
Mg Mg Error P P Error As As Error
300 °C 0.24 0.24 0.14 0.03 0.22 0.00
500 °C 0.94 0.22 0.13 0.02 0.33 0.01
700 °C 0.00 0.11 0.12 0.01 0.07 0.01
5.5 Differential scanning calorimetry (DSC)
To determine the beginning of the phase transforma-
tions in air, differential scanning calorimetry was used.
Figure 6 has a DSC heating curve for a run carried out
using a heating/cooling rate of 10 °C/min on a sample of
pure pyrite.
Pyrite was first transformed into an iron(III)sulfide,
which then further dissociated into hematite. We believe
that the disintegration process took place according to
the following chemical reactions in Equations (1), (2)
and (3):
2FeS2(s) + 7O2(g)  Fe2(SO4)3(s) + SO2(g) (1)
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262 Materiali in tehnologije / Materials and technology 51 (2017) 2, 259–265
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Figure 6: DSC heating curve for pyrite in air
Slika 6: DSC-krivulja segrevanja pirita na zraku
Figure 5: XRD analysis of a sample annealed at 700 °C for 4 h
Slika 5: Rentgenogram vzorca `arjenega 4 h na temperaturi 700 °C
Fe2(SO4)3(s)  Fe2O3(s) + 3SO3(g) (2)
SO2(g) + 0,5O2(g)  SO3(g) (3)
Apparently, the disintegration of iron(III)sulfide is
obviously complicated as it involves three partial reac-
tions. So, the total rate of disintegration or conversion of
iron(III)sulfide into hematite depends on the partial rate
of each individual reaction listed above.
5.6 Annealing pyrite in a protective atmosphere
(argon) at 200 °C, 300 °C and 500 °C
Annealing in a protective atmosphere at 500 °C does
not lead to the transformation of pyrite. The Figure 7
shows a diffraction pattern of a sample after annealing at
500 °C for 4 h.
5.7 Annealing pyrite in a protective atmosphere
(argon) at 700 °C
Already after one hour of sample annealing at 700 °C
it can be observed that the crystalline structure of pyrite
transformed to the magnetic pyrrhotite Fe7S8. Figure 8
has a diffraction pattern of magnetic pyrrhotite and the
corresponding unit cell as generated by CrystalMaker.
Additionally, there is the diffraction pattern of a sample
made after annealing at 700 °C for 4 h.
The basic cell of magnetic pyrrhotite Fe7S8, Figure
8c, belongs to the trigonal crystal system, point group
(HM) 3 (3-fold axis with the inversion in the direction of
the a axis), and the space group R3 (rhombohedral lattice
R, and the 3-fold axis with the inversion in the direction
of the a axis).
5.8 The X-ray fluorescent spectroscopy (argon)
Table 2 presents the weight percentages of chemical
elements and the relative error in the calculation of the
weight fraction of each element. Presented are the three
samples that were heated to (300, 500 and 700) °C and
exposed for 4 h in argon.
Table 2: Chemical composition of samples annealed at (300, 500 and
700) °C in argon (X-ray fluorescent spectroscopy, w/%)
Tabela 2: Kemijska sestava vzorcev `arjenih v za{~itni atmosferi (Ar)
pri temperaturah 300 °C, 500 °C in 700 °C (rentgenska fluorescen~na
spektroskopija, w/%)
Tempe-
rature
Chemical element
Fe Fe Error S S Error Si Si Error
300 °C 35.47 0.03 32.25 0.02 0.71 0.07
500 °C 35.41 0.07 32.12 0.01 0.78 0.11
700 °C 47.18 0.02 23.29 0.02 2.98 0.07
Al Al Error Ca Ca Error Cl Cl Error
300 °C - 0.07 0.02 0.07 0.04 0.06
500 °C 0.16 0.13 0.02 0.03 0.05 -
700 °C 0.33 0.03 0.07 0.05 0.05 -
Cr Cr Error Mg Mg Error As As Error
300 °C 0.02 0.00 - 0.09 0.08 -
500 °C 0.02 0.00 0.88 0.05 0.08 0.00
700 °C 0.03 0.00 0.01 0.00 0.00 0.00
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Figure 8: XRD analysis of a sample annealed at 700 °C for 4 h: a) diffraction pattern of magnetic pyrrhotite, b) with its unit cell and c) the
diffraction pattern of the sample
Slika 8: Rentgenogram vzorca `arjenega 4 h pri temperaturi 700 °C: a) generiran rentgenogram magnetnega pirotita, b) z osnovno celico in
c) rentgenogram `arjenega vzorca
Figure 7: XRD analysis of a sample annealed at 500 °C for 4 h
Slika 7: Rentgenogram vzorca `arjenega 4 h na temperaturi 500 °C
As can be seen from Table 2, we have a sample of
about 35 wt. % of Fe and about 32 wt. % of S. Since the
X-ray fluorescence spectroscopy cannot detect light
chemical elements, from H to Na, these could have not
been analyzed. Hence, the mass percentages of the
remaining chemical elements do not sum up 100 wt. %.
The difference can be attributed to the presence of light
elements.
5.9 Differential scanning calorimetry (DSC)
To determine the beginning of the phase transforma-
tions in a protective atmosphere we again turned to
differential scanning calorimetry. In Figure 9 the DSC
heating curve for the sample of pyrite was carried out
using a temperature program of 10 °C/min in argon.
From the heating DSC curve we can see that the
change in the crystalline structure of pyrite (Pa3) into
the magnetic pyrrhotite (R3) started at about 530 °C. The
transformation of pyrite in a protective atmosphere takes
place according to the following chemical reaction in
Equation (4):
7FeS2(s)  Fe7S8(s) + 3S2(g) (4)
6 CONCLUSIONS
Based on the results of the X-ray fluorescence spec-
troscopy, X-ray phase analysis and differential thermal
analysis, we can conclude the following:
• The largest amount of impurities in the pyrite sam-
ples was represented by quartz and some heavier
elements, but these were present with a very low
content.
• The process of pyrite decomposition in the oxide
atmosphere depends heavily on the temperature.
• The transformation of pyrite at a temperature of 500
°C takes place gradually by the formation of
iron(III)sulphate, which is further oxidized into the
crystal structure of hematite.
• At a temperature of 700 °C and the oxidizing atmo-
sphere pyrite directly decomposed into hematite after
only 1 h.
• The result of the dissolution of pyrite in a protective
atmosphere is the formation of magnetic pyrrhotite
Fe7S8. Direct transformation occurred already after
1 h of annealing at 700 °C.
• Finally, we established that pyrite is not stable up to
744 °C, as predicted by the Fe-S phase diagram, but
rather undergoes changes into magnetic pyrrhotite.
Acknowledgement
I would like to thank all in the Dept. of Materials and
Metallurgy and the Dept. of Geology who helped me in
this project, especially in the preparation of samples and
the interpretation of results.
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264 Materiali in tehnologije / Materials and technology 51 (2017) 2, 259–265
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Figure 9: DSC heating curve of pyrite in argon
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