Two types of halloysite collected from the upper (UPS) and lower (LOS) zones of a weathered pegmatite profile in the Thach Khoan area, Phu Tho were defined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermal analysis (TG and DTG), and N2
adsorption-desorption isotherms. XRD analysis showed that halloysite and kaolinite coexist in samples of size fractions <2 µm. Semi-quantitative analysis by XRD after formamide (FA) treatment indicated that the halloysite contents are approximately 81% and 93% in UPS and LOS samples, respectively. The results of SEM and TEM analyses showed that while short halloysite type is mainly distributed in the upper zone, long halloysite type occurs primarily in the lower zone of the weathered pegmatite profile. The length of short halloysite ranging from 250 to 750 nm is most popular, accounting for 47.2% of halloysites in the UPS sample. Meanwhile, long halloysites which have the length of 750–1250 nm are dominant in the LOS sample with 69.9%. In addition, short halloysites with outer diameter of >100 nm constitute 79.1% of halloysites in the UPS sample while long halloysites with outer diameter of 50–100 nm make up 74.2% of halloysites in LOS sample. Specific surface areas are 15.7434 and 22.0211 m2
/g and average pore sizes are 18.9837 and 17.0281 nm for the UPS and LOS samples, respectively. The analysis implies that although forming under same natural geographical and climatic conditions, halloysites at different depths in the weathered pegmatite profile may have different morphological and other properties.
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