Insights into the high-pressure behavior of kaolinite from infrared spectroscopy and quantum-mechanical 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 diVraction (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 diVerent 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 ( = 3,595 cmi1 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 cmi1 band to 3,613 cmi1. Finally, kaolinite-I is recovered at 0.6 GPa, conWrming the kaolinite-III!kaolinite-I transformation previously observed by X-ray diVraction, and the irreversibility of the kaolinite-II!kaolinite-III transformation. The ambient spectra collected at the start and Wnish of the experiment are those of kaolinite-I, and start/Wnish band frequencies agree to within 6 cmi1.