Insights into the high-pressure behavior of kaolinite from infrared spectroscopy and quantum-mechanical calculations Welch, M. D. Montgomery, W. /Balan, Etienne Lerch, P. Kaolinite Polytypism In situ high-P Synchrotron IR Phase transformations First-principles calculations 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: 651654, 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: 651654, 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 (v = 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). 2012 text https://www.documentation.ird.fr/hor/fdi:010055654 fdi:010055654 Welch M. D., Montgomery W., Balan Etienne, Lerch P.. Insights into the high-pressure behavior of kaolinite from infrared spectroscopy and quantum-mechanical calculations. 2012, 39 (2), 143-151 EN