A possible terrestrial analogue for haematite concretions on Mars.

Recent exploration has revealed extensive geological evidence for a water-rich past in the shallow subsurface of Mars. Images of in situ and loose accumulations of abundant, haematite-rich spherical balls from the Mars Exploration Rover 'Opportunity' landing site at Meridiani Planum bear a striking resemblance to diagenetic (post-depositional), haematite-cemented concretions found in the Jurassic Navajo Sandstone of southern Utah. Here we compare the spherical concretions imaged on Mars to these terrestrial concretions, and investigate the implications for analogous groundwater-related formation mechanisms. The morphology, character and distribution of Navajo haematite concretions allow us to infer host-rock properties and fluid processes necessary for similar features to develop on Mars. We conclude that the formation of such spherical haematite concretions requires the presence of a permeable host rock, groundwater flow and a chemical reaction front.

Calculated solubility of chrysotile asbestos in physiological systems.

Chrysotile asbestos fibers which are ingested into a physiological system will dissolve in fluids to which they are exposed. The rate of dissolution can be estimated by formulating an appropriate rate law and retrieving rate constants from published experiments. The rate of dissolution calculated for asbestos fibers is dependent on the surface area of dissolving mineral, mass of solution to which the fibers are exposed within cells, the rate of transport of H+ into the cells, and the rate of transport of Mg2+ and SiO2 out of the cells. The maximum rate of dissolution of a single 10-micron X 1-micron fiber within a cell is 2.13 X 10(-7) moles hr-1 kg-1 consistent with dissolution rates measured in vivo by Morgan et al. (A. Morgan, A. Holmes, and C. Gold. Studies of the solubility of constituents of chrysotile asbestos in vivo using radioactive tracer techniques. Environ. Res. 4, 558-570, 1971).

Fiber contamination of vermiculites: a potential occupational and environmental health hazard.

Vermiculite ores from Montana, Virginia, and South Africa have been analyzed for the presence of amphibole contamination. Fibrous actinolite was found in unexpanded Montana vermiculite ore at a maximum concentration of 2.0%. The fibers persisted in the expanded ore at a maximum concentration of 0.6%. Actinolite was also found in the Virginia vermiculite ore but at a lower concentration and mostly as cleavage fragments with low length-to-width ratios. South African ore contained rare anthophyllite fibers also with low length-to-width ratios. Vermiculite ores have the potential for amphibole contamination and can represent potential health hazards without proper occupational and environmental control measures.