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| Titel |
Experimental replacement of aragonite by hydroxyapatite |
| VerfasserIn |
A. Kasioptas, C. Perdikouri, T. Geisler, A. Putnis |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027744
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| Zusammenfassung |
| Hydrothermal treatment of aragonite with (NH4)2HPO4 solution has been shown to produce
hydroxyapatite (HAP) with an overall identical and thus pre-determined morphology
(Kasioptas et al, 2008). The preservation of the morphology is an outcome of the
pseudomorphic nature of this particular reaction. We have investigated the mechanism of the
replacement of aragonite by HAP using single, natural, inorganic aragonite crystals.
Isothermal experiments were carried out with small crystals placed and sealed in autoclaves
with (NH4)2HPO4 solution. After the experiments the aragonite crystals are partly replaced
by a new phase. X-ray powder diffraction confirmed that the product phase is indeed HAP;
however electron microprobe measurements revealed that the HAP is probably
non-stoichiometric. Even when completely converted to HAP, scanning electron
microscopy showed that the fine-structure of the aragonite has perfectly been retained. It
was also observed that the HAP product phase exhibited a high porosity and was
separated from the aragonite parent phase by a sharp interface (on the micrometer
scale).
The replacement of aragonite by HAP in an aqueous solution can be described in terms of
a coupled dissolution-reprecipitation mechanism that takes place at an inward moving
reaction front (Putnis& Putnis, 2007). The porosity in the HAP product phase allows the
solution to reach the reaction interface.
In addition, we have performed experiments with (NH4)2HPO4 solutions prepared with
H2O enriched with 97 at.% 18O. The solutions were pre-heated separately to equilibrate
the oxygen isotopes in the solution. Raman spectroscopy of the HAP product was
used to identify the different vibration modes in the PO43-molecule due to the
exchange of 16O with 18O atoms. Apart from the main ν1(PO4) band located near 962
cm-1, we observed four new bands near 945, 931, 919 and 908 cm-1. We have
attributed the generation of these new bands to four different degrees of 18O atomic
substitutions in the PO43- molecule. The bands that show the highest intensity are those
corresponding to three and four 18O atoms substituting for 16O in the PO4 molecule. A
non-equilibrated phosphate solution was also used in the replacement experiments in order to
observe the simultaneous processes of replacement and H218O-P16O4 exchange
in solution. Differences in the intensity of the four vibration modes offer kinetic
information on the replacement mechanism when compared with kinetic data of the
H218O-P16O4 exchange reaction obtained from in situ Raman investigations (Geisler et al.,
2008).
Geisler T., Kasioptas A., Menneken M., Perdikouri C., Putnis A, Journal of Geochemical
Exploration (2008)
Kasioptas A., Perdikouri C., Putnis C.V., Putnis A, Mineralogical Magazine 72, 77-80
(2008)
Putnis A., Putnis C.V, Journal of Solid State Chemistry 180, 1783-1786 (2007) |
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