Abstract: A working definition for allophane includes amorphous silica and alumina that are quickly soluble in boiling n/2 NaOH. Kinetic studies are used to differentiate and to make a rough estimation of the amount of allophane and certain other impurities present in some kaolin minerals.
Solution rate studies on opal, gibbsite, two allophanes, volcanic ash, quartz, two halloysites, and two kaolinites are presented and in most cases the reaction rate orders with respect to the hydroxide treatment are determined. No interpretation of the observed rate orders are presented.
The decomposition of the two kaolinites followed a third order rate law and their rate constants varied approximately by a factor of ten. The decomposition of the two halloysites followed a second order rate law and their rate constants varied approximately by a factor of five. It is not known why specimens of the same mineral species from different localities should have such large variations in their dissolution rates. Differences in their degree of crystallinity and the resulting surface area may be responsible.