Geological controls on kaolin particle shape and consequences for mineral processing

The kaolinized granites of St. Austell, England, are worked to produce a ra nge of china clay products, for some of which the kaolin has to meet string ent particle shape and size specifications. Systematic petrographic study i ndicates that kaolin occurs in the form of two textural types: (i) finely c rystalline kaolin (typically <5 ($) under tilde mu m in average diameter), which infills dissolution porosity of granitic feldspars, and (ii) coarsely crystalline vermiform aggregates (up to 100 mu m or more in length), which are closely associated with expanded micas. The vermiform aggregates are c haracterized by an intergrowth of mica and kaolin crystals, which can be ob served at scales of resolution offered by TEM. Textural and chemical eviden ce suggest that the expanded mica texture is probably the result of prefere ntial precipitation of kaolin along mica cleavage planes and is not simply a process of chemical replacement. Petrographic examination of kaolin slurries sampled at different points in a typical refinery circuit indicates that platy products with high aspect r atio are derived exclusively from raw materials rich in vermiform aggregate s. The fine scale intergrowth of kaolin and mica within the aggregates resu lts in circumstances where mica persists through to fine grained products. Furthermore, the absence of Fe or other chemical components in the kaolin s tructure suggests that any iron reported for the final products may be a co nsequence of the presence of Fe-bearing mica within a very fine grained int ergrowth.