『Abstract
The crystallographic diversity of pyrrhotite (Fe1-xS),
one of the most common iron sulfide minerals, offers insights
into how mineral-fluid interactions are controlled by crystal
structures. We have conducted oxidative dissolution experiments
on monoclinic 4C-pyrrhotite and ‘hexagonal’ NC-pyrrhotite in aqueous
H2O2/H2SO4 and FeCl3/HCl media at
pH between 1.8 and 2.9 using polished surfaces of single crystals.
Quantification and detailed characterization of the reaction interfaces
has been accomplished by confocal 3D topometry and transmission
electron microscopy (TEM) in conjunction with focused ion beam
(FIB) preparation. Crystallographically coherent intergrowths
of 4C- and NC-pyrrhotite in a single sample allowed unambiguous
identification of strong intrinsic reactivity differences between
the two closely related phases. On {110} faces in the H2O2 medium at 35℃ and pH below 2.70, NC-pyrrhotite
(N〜4.85) reacts about 50-80% faster than 4C-pyrrhotite. Above
pH 2.70, the behavior inverts and 4C-pyrrhotite dissolves faster,
while overall reaction rates drop drastically by up to two orders
of magnitude. Because the two pyrrhotite phases show only marginally
different Fe/S ratios but substantial differences in structural
complexity with regards to vacancy ordering, we attribute the
reactivity differences to structurally controlled processes at
the mineral-water interface. The transition at pH 2.70 is close
to the reported isoelectric point of pyrrhotite. We attribute
the pH dependent changes in reaction rates and behaviors to protonation/deprotonation
of surface sulfhydryl groups and related changes in speciation
and bonding mode of reactive oxygen species at the mineral interface.
At pH<2.70, we find elemental sulfur as a frequent reaction product
in H2O2 and FeCl3 media, indicating incomplete sulfur oxidation.
Above pH 2.70, elemental sulfur was not found in H2O2 experiments (no data for FeCl3).
Our results show that the effects of crystal anisotropy are strong
and directional preference of dissolution changes at the pH 2.70
transition point as well, leading to complex sub-μm-scale textural
development at the reaction interfaces throughout the pH range
studied. High resolution TEM imaging of cross sections through
reacted mineral surfaces show crystalline pyrrhotite up to the
reaction interface and the absence of significant non-equilibrium
layers or S-enriched (poly)sulfides.』
1. Introduction
1.1. General introduction
1.2. Mineralogy of pyrrhotite
1.3. Reactions at pyrrhotite surfaces
1.4. Outline of approach
2. Samples and experimental procedure
2.1. Sample description
2.2. Sample preparation
2.3. Experimental setup and conditions
2.4. Analytical methods
3. Results
3.1. General observations and identification of alteration
phases
3.1.1. Experiments with FeCl3 solution
- surface mineralogy
3.1.2. Experiments with FeCl3 solution
- pyrrhotite reactivity and experimental reproducibility
3.1.3. Experiments with H2O2
solution - surface mineralogy
3.2. Phase-specific quantification of dissolution rates from
H2O2 experiments
3.2.1. Reactivity of NC-pyrrhotite versus 4C-pyrrhotite
3.2.2. Effect of additional solutes
3.3. Orientation dependence of pyrrhotite dissolution
3.3.1. Reaction at pH 2.05
3.3.2. Reaction at pH 2.88-2.92
3.3.3. Control of superstructure on dissolution anisotropy
3.3.4. Anomalous dissolution behavior in microenvironments
3.4. Interface morphology
3.5. High resolution TEM interface observations
4. Discussion
4.1. pH dependence of dissolution rates and anisotropy
4.1.1. Dissolution rates and the isoelectric point
4.1.2. Dissolution anisotropy and chemical state of pyrrhotite
surfaces
4.1.3. Oxidant species
4.1.4. Non-oxidative dissolution and the role of Fe3+
4.2. Reactivity differences between 4C- and NC-pyrrhotite
4.3. Existence of sulfidic non-equilibrium layers
5. Conclusions
Acknowledgements
References