Pyrite
is a ubiquitous mineral in reducing environments and is
well-known to incorporate trace elements such as Co, Ni, Se, Au, and
commonly As. Indeed, As-bearing pyrite is observed in a wide variety
of sedimentary environments, making it a major sink for this toxic
metalloid. Based on the observation of natural hydrothermal pyrites,
As–I is usually assigned to the occupation of tetrahedral
S–I sites, with the same oxidation state as in arsenopyrite
(FeAsS), although rare occurrences of AsIII and AsII have been reported. However, the modes of As incorporation
into pyrite during its crystallization under low-temperature diagenetic
conditions have not yet been elucidated because arsenic acts as an
inhibitor for pyrite nucleation at ambient temperature. Here, we provide
evidence from X-ray absorption spectroscopy for AsII,III incorporation into pyrite at octahedral FeII sites and
for As–I at tetrahedral S–I sites
during crystallization at ambient temperature. Extended X-ray absorption
fine structure (EXAFS) spectra of these As-bearing pyrites are explained
by local structure models obtained using density functional theory
(DFT), assuming incorporation of As at the Fe and S sites, as well
as local clustering of arsenic. Such observations of As–I incorporation at ambient temperature can aid in the understanding
of the early formation of authigenic arsenian pyrite in subsurface
sediments. Moreover, evidence for substitution of AsII,III for Fe in our synthetic samples raises questions about both the
possible occurrence and the geochemical reactivity of such As-bearing
pyrites in low-temperature subsurface environments.
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