MINERALOGY AND CATION EXCHANGE PROPERTIES OF LIBBY VERMICULITE SEPARATES AS AFFECTED BY PARTICLE-SIZE REDUCTION*

Abstract--A vermiculite (Libby, Montana) sample obtained from he Zonolite company contained mostly coarse-grained separates wi h only 8.7 per cent clay. The 2-50 #m fraction was used for particle-size reduction studies by wet and dry grinding, and size-fractionated into < 2, 2-5, 5-20 and 20--50 #m sizes. About 18 per cent of the sample was attrited to clay after 64 h of wet grinding, but as much as 59 per cent of the sample was attrited to clay after only 10 min of dry grinding. There was no evidence of damage to the crystal structure of derived clays or silts except for th 20-50 gm fraction from dry grinding. The observed CEC values of all the fractions decreased as grinding progressed, except for the 2-5 #m fraction from wet grinding where the CEC increased. The decrease inCEC was attributed to an accumu-lation of biotite, either as a discrete mineral and/or a mixed-layer assemblage of biotite and vermiculite attrited to he clay fraction. I contrast, he fraction showing an increase inCEC was due to an increased concentration f higher charge-density (CEC)vermiculite. Biotite-free CEC data for vermiculite suggested that, in general, the coarser vermiculite separates had a higher CEC than the finer ones. The susceptibility of minerals in theLibby vermiculite to cleavage by grinding was: vermiculite> hyd-robiotite> biotite