Effect of dry grinding and leaching on polytypic structure of pyrophyllite

Abstract

The polytypic structure of a purified pyrophyllite sample from Zalamea (Badajoz, Spain) and the structural alteration induced after dry grinding and leaching were studied by means of infrared spectroscopy, nitrogen adsorption, thermal analysis and by X-ray diffraction methods (reflection and transmission). Simulated X-ray powder diffraction patterns, using preparations with random and preferred orientations with coefficients of orientation g=1 and g=3, were compared with the experimental patterns. It was found that the pyrophyllite is in the polytypic modification 1Tc (1A A-II, 1). As compared to those in simulated diffractograms, the diminished intensity of 11 l and 02 l reflections is due to random translations along the b axis. A short grinding time caused further degradation of the 11 l and 02 l diffractions. This leads to disordered forms and not to the formation of a monoclinic polytype as has been claimed previously. DTA showed a temperature shift in the main endothermic peak after grinding. This shift was fastest during the first few minutes of grinding. In addition, DTA peak areas decreased with increasing grinding time and with increasing surface area of the ground silicate. These changes are related to a progressive delamination and structural breakdown during grinding. Prolonged grinding, up to 30 min, created a turbostratic-type structure and a drastic reduction in particle size to about 0.04 μm in diameter and about 0.02 μm in thickness, this size reduction being related to the thermal features. After 30 minutes of grinding the mechanochemical reduction of the original particles appears to have reached a limit. At longer times there is an increasing degree of amorphization and a decrease in the surface area by reaggregation of the ground powder. Thus, the thermal behaviour is strongly dependent on the structural state of the material. The polytypism of pyrophyllite is discussed. The X-ray results are compared with those for other pyrophyllite samples (Honami, Japan, the former Yugoslavia, Venezuela and the Ukraine). It is concluded that the formation of a one-layer triclinic polytype of pyrophyllite or disordered forms is more likely than the formation of the two-layer monoclinic variety.