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| Titel |
The role of grain boundaries and transient porosity increase as fluid pathways for reaction front propagation |
| VerfasserIn |
Laura Jonas, Timm John, Thorsten Geisler, Andrew Putnis |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076205
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| Zusammenfassung |
| The pseudomorphic replacement of Carrara marble by calcium phosphates was studied as a
model system to examine the influence of different fluid pathways for reaction front
propagation induced by fluid-rock interaction. In this model system, the grain boundaries
present in the rock and the transient porosity structures developing throughout the
replacement reaction enable the reaction front to progress further into the rock as well as to
the center of each single grain until complete transformation. Hydrothermal treatment of the
marble using phosphate bearing solutions at temperature levels of 150Ë C and 200Ë C
for different durations lead to the formation of two product phases which were
identified as hydroxyapatite [Ca5(PO4)3OH] as well as β-tricalcium phosphate
[β-Ca3(PO4)2] (β-TCP). The formation of β-TCP was probably favored by the presence of
~0.6wt.% of Mg in the parent phase. Completely transformed single grains show a
distinctive zoning, both in composition and texture. Whereas areas next to the grain
boundary consist of nearly pure hydroxyapatite and show a coarse porosity, areas close
to the center of the single grains show a high amount of β-TCP and a very fine
porous microstructure. If F was added as an additional solution component, the
formation of β-TCP was avoided and up to 3wt.% of F were incorporated into the
product apatite. The use of the isotope 18O as a chronometer for the replacement
reaction makes it possible to reconstruct the chronological development of the calcium
phosphate reaction front. Raman analysis revealed that the incorporation of 18O
in the PO4 tetrahedron of hydroxyapatite results in the development of distinct
profiles in the calcium phosphate reaction front perpendicular to the grain boundaries
of the marble. Through the use of the 18O chronometer, it is possible to estimate
and compare the time effectiveness of the different fluid pathways in this model
system. The results show that the grain boundaries serve as a very effective pathway
that enable the fluid to penetrate the rock more than one order of magnitude faster
compared to the newly developing channel-like porosity structures which act as
pathways towards the center of single mineral grains. Thus, it may be possible for the
fluid to progress relatively large distances along the grain boundaries after only
short reaction durations without producing broad reaction fronts along the path. |
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