Field spectroscopy applied to the kaolinite polytypes identification

A detailed characterization of some minerals of the kaolinite group has been done. The
mineralogical and structural characterization has been conducted by X-ray diffraction (XRD)
together with the study of the spectroscopy response in the visible-near infrared and short wave
(VNIR-SWIR), and the main objective was the determination of kaolinite polytypes. For this
purpose, 13 samples were selected from a wide group of samples.

X-ray diffraction patterns confirm the high purity of all the samples which only present scarce
amounts of quartz or feldspar in certain cases. The detailed study of XRD patterns allows
identifying characteristic reflections and small features of each polytype as well as to obtain
crystallinity indices for each sample. The statistical analysis of the patterns groups the samples
according the kaolinite polytype in 5 kaolinites, 2 dickites and 6 halloysites.

The spectroscopic study (VNIR-SWIR) of kaolin samples shows different features in the
wavelength range studied with the ASD FieldSpec 4 Standard-Res. Groups of diagnostic peaks
for kaolinite, dickite y and halloysite polytypes have been identified in the second derivative of
the spectrum, of which position and intensity have been treated statistically with the aim of
classifying the spectra according to the polytype. In good agreement to the XRD results, the
statistical analysis of the spectroscopic data, both by cluster analysis and by principal
components analysis (PCA), allow an unequivocal classification of the samples according to the
polytype from their VNIR-SWIR spectra.