Insights into the high-pressure behavior of kaolinite from infrared spectroscopy and quantum-mechanical calculations |
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Authors: | Email author" target="_blank">Mark?D?WelchEmail author Wren?Montgomery Etienne?Balan Philippe?Lerch |
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Institution: | 1.Department of Mineralogy,The Natural History Museum,London,UK;2.Department of Earth Sciences and Engineering,Imperial College,London,UK;3.Institut de Minéralogie et Physique des Milieux Condensés (IMPMC), UMR CNRS 7590, UMR IRD 206,Université Paris VI,Paris,France;4.Swiss Light Source,Paul Scherrer Institut,Villigen PSI,Switzerland |
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Abstract: | The high-pressure behavior of Keokuk kaolinite has been studied to 9.5 GPa by infrared spectroscopy using synchrotron radiation.
The kaolinite-I → kaolinite-II and kaolinite-II → kaolinite-III transformations have clear spectroscopic expression, with
discontinuities coinciding with the transformation pressures bracketed by X-ray diffraction (Welch and Crichton in Am Mineral
95:651–654, 2010). The experimental spectra have been interpreted from band assignments derived from density functional theory for the structures
of kaolinite-II and kaolinite-III, using as starting models the ab initio structures reported by Mercier and Le Page (Acta
Crystallogr A B64:131–143, 2008, Mater Sci Technol 25:437–442, 2009) and unit-cell parameters from Welch and Crichton (Am Mineral 95:651–654, 2010). The relaxed theoretical structures are very similar to those reported by Mercier and Le Page (Acta Crystallogr A B64:131–143,
2008, Mater Sci Technol 25:437–442, 2009) in their theoretical investigation of kaolinite polytypes at high pressure. The vibrational spectra calculated from the
quantum-mechanical analysis allow band assignments of the IR spectra to be made and provide insights into the behavior of
different OH environments in the two high-pressure polytypes. The single perpendicular-interlayer OH group of kaolinite-III
has a distinctive spectroscopic signature that is diagnostic of this polytype (ν = 3,595 cm−1 at 9.5 GPa) and is sensitive to the compression/expansion of the interlayer space. This OH group also has a distinctive signature
in the calculated spectra. The spectra collected on decompression are those of kaolinite-III and persist largely unchanged
to 4.6 GPa, except for a continuous blue shift of the 3,595 cm−1 band to 3,613 cm−1. Finally, kaolinite-I is recovered at 0.6 GPa, confirming the kaolinite-III → kaolinite-I transformation previously observed
by X-ray diffraction, and the irreversibility of the kaolinite-II → kaolinite-III transformation. The ambient spectra collected
at the start and finish of the experiment are those of kaolinite-I, and start/finish band frequencies agree to within 6 cm−1. |
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