Morphological change of halloysite derived from weathering of illite in residual kaolin

The microtextures of vermicular grains formed by the weathering of illite to kaolin minerals in residual kaolin derived from anorthosite were examined using transmission electron microscopy to clarify their mineralogical make-up and formation process. In the vermicular grains, platy kaolin minerals had replaced illite from the grain edge to the interior in a pseudomorphic pattern. Remnants of illite were intermittently distributed within the vermicular grains, where illite packets transformed to kaolin packets with little volume change. The width of the packets of kaolin minerals was highly uniform, at between 50-80 Å, and was comparable to those found in a typical halloysite. The kaolin packets were cleaved androlled into tubes at the edges of the vermicular grains, and often included remnants of illite packets, which were dispersed as individual tubes by exfoliation. We strongly suggest that the platy kaolin mineral replacing illite is platy halloysite, which has been rolled into tubes in the free spaces to release inherent strain.