Recognition of intermediates in the possible transformation of halloysite to kaolinite in weathering profiles

The kaolin minerals in lateritic weathering profiles on dolerite and granite in Western Australia have been characterized by intercalation tests for halloysites, and also by their particle morphologies. Towards the surface of the profiles there is a decrease in both total kaolin mineral content, and in the expandibility of those minerals to intercalation. Tubular kaolinite that is non-expandible by any of the intercalation tests for halloysite was identified in the upper few metres of both profiles. Hydrated halloysite is most common at the base of both profiles. Formamide expands some anhydrous kaolin minerals while the sequence of hydrazine/water/glycerol (HWG) expands more of these minerals. Non-tubular (subhedral to sub-hexagonal platy) kaolinite is also present. Hydrated tubular halloysite apparently alters to tubular kaolinite via dehydrated halloysite that is expandible by formamide and a tubular ‘kaolin’ that is expandible by HWG but not by formamide.