『Abstract
In situ dissolution experiments on a set of pure, optical quality
Iceland spar calcite samples from four different localities showed
etch pit step retreat rates to be inversely proportional to total
inherent trace cation composition. Atomic absorption spectroscopy
(AAS) revealed Fe2+, Mg2+, Mn2+
and Sr2+ in amounts varying from a few to hundreds
of ppm. We used a very simple experimental setup, with an Atomic
Force Microscope (AFM) fluid cell and a droplet of MilliQ water.
As the calcite dissolved and approached equilibrium with the solution,
trace cations were released, which were then present for interaction
with the dissolving surface. We monitored continuous free--drift
dissolution, in situ, on fresh {1014}(後の1の頭に-)
cleavage surfaces for up to 40 min. Dissolution produced one-layer-deep,
rhombic etch pits that continually expanded as we collected images.
The rhombohedral symmetry of calcite defines two obtuse and two
acute edges on the cleavage surface of etch pits and these, as
expected from previous work, had different dissolution rates.
Despite identical experimental conditions for all samples, we
observed lower step retreat rates for both obtuse and acute edges
on calcite characterised by relatively high trace cation composition.
Increased cation concentration, particularly Mn, was also correlated
with rounding of obtuse-obtuse corners, resulting in obtuse step
retreat rates similar to those for acute sides. Physical limitations
of the AFM technique were taken into account when measuring step
rate retreat and results were collected only from single-layer
etch pits, which represent crystalline calcite with minimal defects.
Dissolution rates presented here are thus lower than previous
reports for studies of deep etch pits and where the physical limitations
of imaging may not have been considered. In addition to molecular-level
proof that divalent cations inherent at ppm levels in the calcite
affect the dissolution processes, these results show that pure,
optical quality Iceland spar calcite should not be considered
pure in the chemical sense. The results imply that dissolution
rates determined for ideal systems with pure, synthetic or natural,
materials may be considered as the boundary condition for dissolution
in real systems in nature, where cations are always present both
in the solution and in the initial solid.』
1. Introduction
2. Experimental details
2.1. Materials
2.2. Atomic absorption spectroscopy (AAS)
2.3. In situ, fluid-cell AFM
2.4. Imaging procedure
2.5. Etch pit terminology
3. Results and discussion
3.1. Trace element composition
3.2. Dissolution experiments
3.2.1. Dissolution behaviour and etch pit morphology
3.2.2. Step retreat rates
3.3. The effect of trace metals on calcite step retreat rates
3.4. Inhibition mechanism
3.5. Comparison with previous work
4. Conclusions
Acknowledgments
References