『Abstract
Pyrite is precipitated within the cells of higher plants and
algae at ambient temperatures by diffusion of aqueous Fe(II) followed
by reaction with microbial or inorganic S(-II). The absence of
any additional oxidizing agent in sufficient concentrations to
account for the mass of pyrite formed in the experimental systems
suggests that the reaction involves aqueous H2S.
Pyrite is formed from undersaturation relative to mackinawite,
FeSm, since FeSm is
not observed in the cells and it is seen to dissolve in the experimental
systems. the overall pyrite forming reaction, starting with Fe(II)
as a reactant, appears to be
Fe(II) + H2S→FeSo + H2S→FeS2p
where FeSo is the monomeric representation of an aqueous
FexSx cluster or reaction
intermediary. Although a series of balanced reactions could be
written, none of these provides a unique description of the process
and reaction mechanisms cannot be determined from this type of
empirical experimentation. Pyrite formation is determined by the
rate of H2S diffusion into the cells. An
apparent diffusion coefficient for H2S of
10-8 cm2 s-1 is obtained, which
includes the effects of tortuosity and effective porosity. The
supersaturation limit for pyrite formation in these cells approaches
1011, which is lower than the value found for pyrite
nucleation on pyrite seeds. Biological surfaces, such as plant
cells, appear to have a greater surface activity than pyrite seeds.
However, the supersaturation limit is affected by transport through
the diffusion boundary layer, which is a surface-independent common
rate-determining process. The rate of pyrite formation in the
cells is relatively rapid, with over 50 vol.% pyrite being formed
within the parenchyma cells of Apium sp. within 2 weeks.
The rapid formation of pyrite within plant cells contributes to
the preservation of cells during fossilization. There is no evidence
for particulate iron sulfides penetrating the cells and the nature
of the cell wall would preclude this. Greigite, Fe3S4 and mackinawite, FeSm,
are therefore not necessarily direct precursors to pyrite formation,
which is consistent with their observed limited abundance in sedimentary
systems. The direct involvement of stoichiometric quantities of
other inorganic components, such as dioxygen and elemental sulfur,
was precluded in the experimental design, through the chemistry
of the plant substrate and through the exclusion of particulates
by the cell wall system. The intracellular chemical environment
is complex, however, and trace oxidants may play a role in initiating
pyrite nucleation.
Keywords: Pyrite; Plant; Fossilization; Sulfide; Celery; FeS』
1. Introduction
2. Methods
2.1. Batch experiments with Chlorella
2.2. Batch experiments with Apium
2.2.1. Experiments with celery strips
2.2.2. Experiments with whole celery petioles
2.3. Chemostat experiments with Apium and Platanus
2.4. Observations of products
3. Results
3.1. Experiments with Chlorella
3.2. Experiments with Apium
3.2.1. Batch experiments
3.2.2. Chemostat experiments
4. Discussion
4.1. Exclusion of precipitated FeSm
4.2. Advection and diffusion
4.3. Mass-volume considerations
4.4. Fe transport in sulfidic systems
4.5. Pyrite nucleation
4.6. Constraints on pyrite formation
5. Conclusions
Acknowledgments
References