| Abstract: | White, pale-yellow and brown deposits occur on surfaces of fragments of the Norton County enstatite achondrite. X-ray powder analysis of these materials indicates that they consist of several calcium-bearing minerals: portlandite [Ca(OH)2], vaterite (CaCO3, hexagonal), calcite (CaCO3, trigonal) and bassanite (CaSO4-1/2 H2O). We suggest that these minerals formed by weathering of oldhamite (CaS), which we found to occur in Norton County. The occurrence of portlandite suggests that at low temperatures and in the terrestrial environment, hydrolysis of oldhamite is the most important first step in the weathering sequence. Subsequent carbonation of portlandite is thought to produce vaterite, and vaterite in turn might transform into calcite. Thus, we suggest a weathering sequence in the terrestrial environment of oldhamite  portlandite  vaterite  calcite. The mineral bassanite is clearly also a terrestrial weathering product but its precise mode of origïn is somewhat uncertain: in the system CaSO4-H2O, bassanite forms from gypsum at about 100 °C (an unlikely high T for the weathering environment of Norton County). |
