『Abstract
Prior transmission electron microscope studies showed that the
surface geometry of olivine changes dramatically during natural
chemical weathering. However, similar morphological evolution
has not been reported in laboratory studies of olivine dissolution.
In this study, we examined the development of fayalite (Fe2SiO4) surface morphology
during both abiotic and biotic (using Acidithiobacillus ferrooxidans)
laboratory dissolution experiments at an initial pH of 2.0. The
fayalite came from Cheyenne Canyon, Colorado (Smithsonian # R
3516) and contains a few percent laihunite (olivine structure
with ordered ferric iron and vacancies, 〜Fe0.82+Fe0.83+SiO4). High-resolution
field emission low voltage scanning electron microscope (SEM)
characterization of all reacted samples showed etch patterns consistent
with those reported from naturally reacted olivine. High-resolution
transmission electron microscope (HRTEM) data demonstrated pervasive
channeling on (001), with channel spacings that range down to
<10 nm. Formation of channel on (001) is probably initiated by
preferential removal of cations from olivine M1 sites. Channeling
confers at least an order of magnitude increase in surface area.
Relict strips of olivine between channels contain laihumite layers
that are oriented parallel to channel margins. X-ray diffraction
analyses indicated that the relative abundance of laihunite is
higher in reacted compared to unreacted samples. This result is
consistent with prior studies of naturally weathered olivine that
suggest that laihunite is far less readily dissolved than olivine.
Samples reacted in the presence of A. ferrooxidans cells
that enzymatically oxidized iron, or in solutions where ferric
iron was added to simulate biological activity, dissolve at a
much slower rate than samples reacted abiotically. We attribute
suppression of the olivine dissolution rate to surface adsorption
of Fe3+. It is probable that ferric iron adsorption
is controlled by M2 sites in the underlying olivine structure.
If this is coupled with removal of M1 cations during channel formation,
then a modified laihunite-like surface will develop (vacancies
in laihunite are on M1 sites). Although surface modification might
only penetrate a few atomic layers, an inherently unreactive laihunite-like
surface structure could explain both the pervasive channeling
and the dramatic suppression of the measured dissolution rate.』
1. Introduction
2. Material and methods
3. Results
3.1. Abiotic experiments
3.2. Biologic experiments
3.3. Naturally weathered fayalite from fayalite granite
4. Discussion
Acknowledgments
References