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Greigite: a true intermediate on the polysulfide pathway to pyrite
Geochemical Transactions
Greigite: a true intermediate on the polysulfide pathway to pyrite Stefan Hunger* and Liane G Benning
0 Address: Earth and Biosphere Institute, School of Earth and Environment, University of Leeds , Leeds, LS2 9JT , UK
The formation of pyrite (FeS2) from iron monosulfide precursors in anoxic sediments has been suggested to proceed via mackinawite (FeS) and greigite (Fe3S4). Despite decades of research, the mechanisms of pyrite formation are not sufficiently understood because solid and dissolved intermediates are oxygen-sensitive and poorly crystalline and therefore notoriously difficult to characterize and quantify. In this study, hydrothermal synchrotron-based energy dispersive X-ray diffraction (ED-XRD) methods were used to investigate in situ and in real-time the transformation of mackinawite to greigite and pyrite via the polysulfide pathway. The rate of formation and disappearance of specific Bragg peaks during the reaction and the changes in morphology of the solid phases as observed with high resolution microscopy were used to derive kinetic parameters and to determine the mechanisms of the reaction from mackinawite to greigite and pyrite. The results clearly show that greigite is formed as an intermediate on the pathway from mackinawite to pyrite. The kinetics of the transformation of mackinawite to greigite and pyrite follow a zero-order rate law indicating a solid-state mechanism. The morphology of greigite and pyrite crystals formed under hydrothermal conditions supports this conclusion and furthermore implies growth of greigite and pyrite by oriented aggregation of nanoparticulate mackinawite and greigite, respectively. The activation enthalpies and entropies of the transformation of mackinawite to greigite, and of greigite to pyrite were determined from the temperature dependence of the rate constants according to the Eyring equation. Although the activation enthalpies are uncharacteristic of a solid-state mechanism, the activation entropies indicate a large increase of order in the transition state, commensurate with a solid-state mechanism.
Background
The formation of pyrite is an important geochemical
pathway linking the global biogeochemical cycles of iron,
sulfur and carbon in anoxic sediments [
1-3
]. Furthermore,
chemical reactions involved in pyrite formation have
important implications for the fate and mobility of toxic
[
4,5
] and radioactive [6] metals in near-surface
environments. Over the past half century, the formation of pyrite
has been studied extensively at low temperatures and
several pathways have been proposed [
1,7-10
]; yet the
mechanisms of pyrite formation in anoxic sediments and the
chemical conditions favoring its formation and stability
are still not fully understood.
It was recognized early that the formation of pyrite from
iron monosulfide precursors in anoxic sediments required
an oxidant [
1
]. In one of the first systematic laboratory
investigations of pyrite formation, Berner [
1
] found that
zerovalent sulfur dissolved as polysulfides oxidized iron
monosulfide and lead to the formation of pyrite at 65°C
(Equ. 1). In addition, he found that the so formed pyrite
grains had similar morphologies to natural pyrite
framboids and suggested that reaction (1) may thus play a
crucial role in most sedimentary environments.
FeS + S0 → FeS2
(1)
Drobner and coworkers [
11
] and later Rickard [
10,12,13
]
proposed that hydrogen sulfide can act as an oxidant of
iron monosulfide, yielding pyrite and hydrogen gas.
Rickard and Luther concluded from polarographic results [
7
]
that aqueous iron monosulfide complexes in equilibrium
with the solid phase react with hydrogen sulfide in
solution, producing pyrite via a dissolution/re-precipitation
pathway [
10
] (Equ. 2).
Schoonen and Barnes [
8
] and Luther [
7
] have suggested
that solid FeS reacts with adsorbed polysulfide via a cyclic
intermediate and a combined nucleophilic/electrophilic
attack to nucleate pyrite. Luther also proposed from his
polarographic results, that a dissolved FeSH+ complex
reacts in a similar fashion with polysulfide, nucleating
pyrite from solution (Equ. 3) [
7
].
that cause these magnetotactic bacteria to be oriented in
magnetic fields.
The investigation of the formation of pyrite from
precursors such as mackinawite (FeS) and greigite is hampered
by the fact that these phases are poorly crystalline and
extremely sensitive to oxidation, which makes
characterization by conventional powder X-ray diffraction (XRD)
difficult. Furthermore, no wet-chemical technique is
available for the distinction between and quantification of
both phases, and they are commonly subsumed in the
pool of "acid-volatile sulfide" (AVS) [
13
].
From a theoretical point of view, greigite can form on the
pathway to pyrite as an intermediate species in the
reaction between mackinawite and excess sulfur (Equ. 4) or
via an iron loss pathway (Equ. 5).
−2e−
3FeS → Fe3S4
+S2−
− 4 (...truncated)