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Microstructural evolution during experimental albitization of K-rich alkali feldspar
Ist Teil von
Contributions to mineralogy and petrology, 2011-09, Vol.162 (3), p.531-546
Ort / Verlag
Berlin/Heidelberg: Springer-Verlag
Erscheinungsjahr
2011
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Crystals of K-feldspar (
C
2/
m
), in contact with highly concentrated aqueous NaCl solutions at 500°C and 200 MPa, are pseudomorphically replaced by high albite
as a result of an interface-coupled dissolution/reprecipitation process. The reaction occurs at an extremely sharp reaction front (<10 nm) and involves the complete breakdown of the initial framework structure. This results in the release of tetrahedrally incorporated elements such as Fe
3+
and Ti
4+
and a significant increase in Si/Al disorder across the reaction interface. The evolving microstructure is controlled by crystallographic relations between the phases. This leads to highly anisotropic, sawtooth-shaped intergrowths of albite and initial K-feldspar, resulting in the least structural misfit between the two framework structures. As a result, the newly formed interfaces appear to be semicoherent, and cracks across the reaction fronts even indicate elastic strain. The reaction produces 2 distinctive albite types (
albite
-
1
and
albite
-
2
). Both are polycrystalline, with
albite
-2 showing significantly larger subgrain sizes. This indicates a secondary coarsening step driven by the reduction in interfacial energy within the polycrystalline replacement product. The reaction also produces a highly porous rim. However, the porosity is not evenly distributed resulting in a porous
albite
-
1
and a non-porous
albite
-
2
that mostly surrounds large, euhedral pores. Despite the substantial volume fraction of porosity in
albite
-
1
, no significant 3D interconnectivity could be detected, making the presence of a pervasive porosity unlikely. However, the result of coarsening is the continuous modification of the 3D porosity distribution. This could potentially provide a mechanism for fluid transport through the replacement rim until textural and chemical equilibration is achieved.