Artificial Transformation of Ground Talc into Kaolinite and Smectites by Al-salt solution

Abstract

Talc has been known as a low reactive mineral under hydrothermal conditions in comparison with the other clays. However, it was expected that grinding treatment increase its reactivity by so-called mechanochemical effect.
In the present study, naturally occurred talc from China was ground in vibrating mill to X-ray amorphous state. The ground talc was then treated hydrothermally with aluminum salts solution for various Al (in reaction systems)/Si (in ground talc) molecular ratios ranging from 0.1 to 2.0. The products were examined by means of X-ray diffractometry, thermal analysis, and infrared absorption spectroscopy.
It was found that the ground talc is successively converted to trioctahedral smectite, dioctahedral smectite, and kaolinite with increasing the Al/Si ratios, by treatment with acidic aluminum bearing solution such as AlCl₃, Al₂(S0₄)₃, and Al (NO₃)₃. Al₂₄O₁₁(OH)₄₄Cl₆ phase and alunite were also formed as the coproducts in the way of AlCl₃ and Al₂(SO₄)₃ solution treatment, respectively.
On the other hand, by the treatment with NaAlO₂, the ground talc changed to a trioctahedral smectite and an analcime at low Al/Si ratios, but a serpentine and a hydroxysodalite were formed at high Al/Si ratios. In these cases, the reaction was proceeded in basic condition, and the pH value was increased with increasing of the Al/Si ratios.
It is concluded that grinding significantly increases the hydrothermal reactivity of talc, and the hydrothermal products of the ground talc are controlled by the Si/Al ratio and pH value in reaction system.