Studies of the weathering of platinum-group minerals in natural deposits have indicated that the platinum-group elements (PGE) can be mobilized under appropriate conditions and have sufficient solubility to move into solution. Although inorganic chemistry provides feasible mechanisms under the extreme conditions encountered in laterites it appears likely that PGE-organic complexes offer a more convincing explanation of the apparent solubility. Experimental work undertaken to illustrate the organic processes and the reaction timescale examined various reactions including those between Pt and Pd foils and humic and fulvic acid solutions. These reactions appear pertinent to those that may occur between the PGE particles ejected by vehicle exhaust catalysts and the natural environment.

The experimental work produced up to 195 μ/l Pt and 102 μ/l Pd in humic acid solutions exposed to the metal foils for about a year at room temperature. The experimental conditions can be compared to those encountered in roadside drainage supplied with PGE particles and decaying vegetation.

Consideration of the possible health effects of the PGE from the particles ejected by vehicle exhaust catalysts has centred on inhalation of the particles or absorption through the skin. Intake from drinking water or from food produced using PGE-bearing water should now also be investigated. Due to the variety of organic compounds produced by decaying vegetation the identity of the PGE-organic products is not yet known so their toxicity cannot be estimated.

New insights from the recent literature are summarized and new data presented concerning the formation, structure and morphology of halloysite. Halloysite formation by weathering always requires the presence of water. Where substantial drying occurs, kaolinite is formed instead. Halloysite formation is favoured by a low pH. The octahedral sheet is positively charged at pH < ∼8, whereas the tetrahedral sheet is negatively charged at pH > ∼2. The opposing sheet charge would facilitate interlayer uptake of H2O molecules. When halloysite intercalates certain polar organic molecules, additional (hkl) reflections appear in the X-ray diffraction pattern, suggesting layer re-arrangement which, however, is dissimilar to that in kaolinite. Associated oxides and oxyhydroxides of Fe and Mn may limit the growth of halloysite particles as does incorporation of Fe into the structure. Particles of different shape and Fe content may occur within a given sample of halloysite.