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Mineral reaction interfaces and associated
porosity generation
Cole D. R., Labotka T. C., Larson P. B., More K. L.,
Kenik E. A., Fayek M., Stadermann F. J., and Riciputi L. R.
(2005) Geochim. Cosmochim. Acta 69,
A169.
ABSTRACT
Chemical processes commonly encountered in nature,
including hydration-dehydration, cation exchange and
oxidation-reduction reactions involving complex fluids
containing electrolytes and mixed-volatiles can lead to
either passivated mineral surfaces or the formation of
reaction rims. Further exchange requires advection and/or
diffusion of matter across the zone to the interface between
the transformed and unreacted parent phase. Structures
within the reaction zone and at the reaction interface, as
well as reaction mechanisms and reaction rates, are still
poorly constrained for most important rock-forming mineral.
This study focused on two aspects of the replacement
process, the geometry of the reaction front and the
generation of nano- and/or microscopic porosity at or near
the interface. Using transmission electron microscopy (TEM)
and secondary ion mass spectrometry (SIMS), we interrogated
the reaction interface domains in two types of feldspar
systems: (a) replacement of plagioclase (~An30)
by albite ± muscovite in the Rico, CO
paleo-hydrothermal system, and (b) Amelia albite replaced by
K-feldspar in experiments (up to 600ºC and 200 MPa)
employing 1-2 molal KCl enriched in 18O. In the
case of the natural system, the typical reaction interface
appears as a curvilinear, somewhat diffuse zone where
crystallographic control is not particularly pronounced.
Micropores occur infrequently along the interface, but are
commonly observed within fine albite-filled fractures and
dislocations that either crosscut or are truncated by the
reaction interface. In contrast, the nature of the reaction
interface in the experimental sample depends strongly on
crystallographic control with an overall jagged appearance,
exhibits limited microporosity, and has an unusual
corrugated or stitch-like nano-texture right at interface.
NanoSIMS images and line scans indicate the interface is
sharp in the distribution of 18O and
16O.
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