Photon Factory Activity Report 2018 #36 (2019)

BL-8B, BL-9C/2018G013

Structural evolution from iron sulfide nanoparticle to pyrite
Yoshinari Sano,* Atsushi Kyono and Sota Takagi
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan

1 Introduction
Iron sulfide minerals plays a major role in controlling theredox chemistry in anaerobic marine and lake sediments.
Sulfide ion (S2-) is reduced from sulfate ion (SO42-) by themetabolic activity of sulfate-reducing bacteria (SRB), but
it is re-oxidized to sulfur (S0) and further oxidized tosulfate ion (SO42-) by the sulfur bacteria (SB) in thesedimentary environments. This biological sulfur cycle isone of the most important elemental cycle at the Earth’ssurface. Iron sulfide (FeS) nanoparticles are initiallyformed by reaction between the ferrous iron (Fe2+) and thesulfide ion (S2-) (Berner 1967). The FeS nanoparticle,
which shows the X-ray amorphous nature, consists ofdisordered arrangement of FeS4 tetrahedral sheets(Wolthers et al., 2003; Ohfuji and Rickard, 2006;
Csákberényi-Malasics et al., 2012). Under the anoxic
aqueous conditions, it is gradually evolved to more stableiron sulfide minerals. By grain growth, the FeS
nanoparticle transforms to mackinawite (FeS), consistingof the FeS4 tetrahedral sheets (Jeong et al., 2008;
Csákberényi-Malasics et al., 2012). By the oxidation offerrous (Fe2+) to ferric (Fe3+) iron, the mackinawite furthertransforms to the thiospinel iron sulfide mineral, greigite(Fe3S4) (Schoonen and Barnes, 1991; Jeong et al., 2008;
Csákberényi-Malasics et al., 2012). By supply of sulfur(S0), the FeS4 tetrahedral coordination changes into FeS6octahedral coordination in the structure, which leads topyrite with the perfect long-range atomic order (Schoonenand Barnes, 1991). Therefore, the pyrite is the mostabundant sulfide mineral at the Earth’s surface (Rickard,2012).
Despite the numerous studies and the importance ofpyrite formation in the biological sulfur cycle, thetransformation mechanism from the FeS nanoparticle topyrite has not been fully understood.
In the present study, hydrothermal experiments and X-ray
crystal structural analysis were performed to clarify thephase transformation and structural change mechanism
from the FeS nanoparticle to pyrite.
2 Experiment
In the study, deoxygenated water was applied in the all
experiments due to the high reactivity of the FeS
nanoparticle toward oxygen. The deoxygenated water wasprepared by bubbling N2 gas to distilled water. First, theFeS nanoparticle suspension was obtained by mixing 10mL of 0.2 M (NH4)2Fe(SO4)2·6H2O aqueous solution with
10 mL of 0.4 M Na2S·9H2O aqueous solution in a glove
box with an inert N2 atmosphere. The obtained blacksuspension was then filtered through a 0.1 µm membranefilter (Merck Millipore Ltd) and blow-dried with N2 gas.
Subsequently, it was kept in a vacuum desiccator until used.
In order to induce the phase transformation from the FeS
nanoparticle to pyrite, the FeS nanoparticle suspension wastransferred into the Teflon container with 77 mg sulfur
powder. The hydrothermal experiments were performed at
120oC for 2, 4, 6, 8, 12, 16, 20, and 24 hours. After heating
for various periods, the resulting suspensions were filteredin the same way and stored in the vacuum desiccator.
Synchrotron powder X-ray diffraction (XRD)
measurements were conducted at BL-8B of PF in KEK. Inorder to avoid any oxidation of the samples, they wereinserted into the glass capillary (φ = 0.7 mm). The
wavelength of X-ray was 0.6868 (5) Å. The exposure time
was set to 30 minutes. The obtained XRD data wereanalyzed with PDIndexer software (Seto et al., 2010).
Fe K-edge X-ray absorption fine structure (XAFS)
measurements were performed at BL-9C of PF in KEK.
The XAFS station was equipped with Si(111) double
crystal monochromator. 4 mg of the sample was diluted
with 150 mg of BN powder and pressed into pellets with adiameter of 10 mm under N2 atmosphere. Ionizationchambers filled with 100% N2 and 15% Ar in N2 were used
to measure the incident and transmitted X-ray intensities,respectively. All spectra were collected in transmission
mode at room temperature. Energy measurement range
was set from 6606.2 to 8211.3 eV. Energy calibration wascarried out using spectrum of Fe foil. The obtained XAFS
spectra were corrected and normalized using the Athenasoftware (Ravel and Newville, 2005). The EXAFS spectra
were imported into Artemis software (Ravel and Newville,2005), and fitted to theoretical models of mackinawite andpyrite using Atoms (Ravel, 2001) and FEFF6 code (Rehret al 1991).
3 Results and Discussion
Figure 1 shows the variation of synchrotron XRD pattern.
At 120oC the FeS nanoparticle was changed to pyrite as afunction of heating time. The FeS nanoparticle, unheatedblack suspension, showed characteristic broad XRDmaxima. The fact that the XRD pattern was nearly thesame to that of mackinawite suggests that the FeS
nanoparticle basically consists of mackinawite-likestructure. This result is approximately consistent with theprevious TEM observation (Ohfuji and Rickard, 2006).
After heating for 8 h, the weak XRD peaks from pyrite andgreigite appeared. After further heating for 16 h, the XRD
peaks from mackinawite completely disappeared. Finally,
only pyrite and greigite were observed. Therefore, most of
the FeS nanoparticle directly changed into the more stable
pyrite by the reaction with sulfur (FeS + S → FeS2), but asmall amount of the FeS nanoparticle transformed togreigite by oxidation of ferrous (Fe2+) to ferric iron (Fe3+).
The previous studies (Rickard and Morse, 2005; Jeong etal., 2008; White et al., 2015) also indicated that greigite
was formed by oxidation of FeS nanoparticle in water
under anoxic solution condition (4FeS + 2H2O → Fe3S4 +Photon Factory Activity Report 2018 #36 (2019)

Fe(OH)2aq + H2). It can be therefore considered that theanoxic water acts as oxidant which oxidizes ferrous ironFig. 1. Variation of XRD pattern as a function ofheating time
(Fe2+) to ferric iron (Fe3+).
Figure 2 shows the variation of radial structure functions(RSF) and fitting results with mackinawite and pyrite
structures. The RSF pattern of FeS nanoparticle, unheatedblack suspension, shows only first shell, which indicatesthat there are only four S atoms around the Fe atom. Thisis, the FeS nanoparticle is solely composed of FeS4
tetrahedra without any long-range atomic order.

As it can be seen in Figure 2, the RSF patterns from 2 to 6
h apparently consist of two shells. The first shellcorresponds to the four S atoms bonded to the central Fe
atom, whereas the second shell is ascribed to the Fe atoms
adjacent to the central Fe atom. This appearance of thesecond shell implies that a long-range order of FeS4
tetrahedra is established by heating. Csákberényi-Malasicset al. (2012) documented that the crystal growth from FeSnanoparticle to mackinawite proceeded under anoxic
hydrothermal condition at 120oC. Our result is therefore ingood agreement with the previous study (CsákberényiMalasics et al., 2012). The RSF patterns from 12 to 24 h
were well fitted with pyrite structure, but the RSF of 8 h
showed an intermediate profile between FeS nanoparticleand pyrite. The FeS nanoparticle transformed gradually topyrite by the reaction with sulfur during heating from 8 to
12 h. The result obtained by RSF patterns seems to be fullycompatible with the XRD measurements.
Figure 3 shows the changes of Fe-S bond distance andcoordination number (CN). In spite of heating time, the FeS bond distance of about 2.26 Å remained unchanged.
Lennie et al. (1995) reported that the Fe-S bond distance inmackinawite was 2.256 Å. On the other hand, thecorresponding bond distance in greigite was 2.147 Å(Skinner et al., 1964). Therefore, the Fe-S bond distance in
the FeS nanoparticle is entirely consistent with that inmackinawite. Fe in FeS4 tetrahedra has a formal valence of+2 in mackinawite, but that in greigite gives an averagevalence of +2.5 by the electron hopping. Since the Fe in
FeS nanoparticle is in the similar tetrahedral coordination
to mackinawite, it also has a formal valence of +2.

Fig. 3. The changes of Fe-S bond distance andcoordination number (CN)Fig. 2. Radial structure functions (RSF)

During heating for 8 h the CN of about 4 was maintained
constant within the experimental error, but it was increased
to 6 after heating for 12 h. This must be ascribed to thecoordination change from FeS4 tetrahedron to FeS6octahedron. That is, the FeS nanoparticle entirelytransformed to pyrite. Since the Fe-S bond distance inpyrite is 2.264 Å (Willeke et al., 1992), the evidence that
the Fe-S bond distance remained unchanged throughout
the heating experiment supports the direct phasetransformation from the FeS nanoparticle to pyrite. In theprevious hydrothermal in-situ ED-XRD study, Hunger andBenning, (2007) reported that at temperatures above
125oC mackinawite transformed to pyrite through greigite
as an intermediate phase. In the present study, however, thepeaks corresponding to greigite was never detected in thePSF patterns because no characteristic short and long Fe-S

Photon Factory Activity Report 2018 #36 (2019)bond distances in greigite (2.147 and 2.464 Å) were clearly
detected. Therefore, the directly phase transformationfrom the FeS nanoparticle to pyrite could preferentiallyoccur at temperatures below 120 oC.
Figure 4 shows the local atomic structure changes fromthe FeS nanoparticle to pyrite. Local atomic structure of
the unheated FeS nanoparticle consists of the FeS4tetrahedron without any long-range atomic order, which
exhibits the X-ray amorphous nature. After heating for 6 h,
the FeS4 tetrahedra started to connect linearly each otheralong one direction. The Fe-Fe atomic distance of 2.69(1)Å in the FeS nanoparticle is however still larger than thatin mackinawite (Fe-Fe = 2.598 Å; Lennie et al., 1995).
This result suggests that the crystal structure of the FeS
nanoparticle is slightly different from that of mackinawite
previously reported. The small angle X-ray scattering
study (Wolthers et al., 2003) and TEM observation (Ohfuji
and Rickard, 2006; Csákberényi-Malasics et al., 2012)
showed that the FeS4 tetrahedral sheets in FeS nanoparticle
are more or less curved. Therefore, the larger Fe-Fe atomic
distance in the FeS nanoparticle could result fromdistortion in the FeS4 tetrahedral sheets. After heating for
12 h, the S-S bonds formation and coordination number
change of Fe occur (Figs. 2 and 3), which indicates theformation of pyrite.

Fig. 4: The local atomic structure changes
4 Conclusions
In the study, the phase transformation and structuralchange mechanism from FeS nanoparticle to pyrite were
investigated by using hydrothermal experiments,synchrotron XRD measurements, and EXAFS analysis.
The result of synchrotron XRD analysis showed that themost of FeS nanoparticle directly transformed to pyrite by
the reaction with sulfur, but a small amount of FeS
nanoparticle was oxidized and transformed to greigite. The
local atomic structure analysis revealed that the FeS
nanoparticle consists of distorted FeS4 tetrahedral sheets.
By heating for 12 h, the FeS4 tetrahedra directly change
into the FeS6 octahedra, which results in the formation ofpyrite. These results provide fundamental data of themineral formation and structural behaviours of the FeS
nanoparticle and pyrite in anaerobic aqueous environments.
Acknowledgement
We sincerely thank Y. Niwa and H. Nitani for their kindassistance and technical advice.

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* sano@geol.tsukuba.ac.jp