CHEMISTRY OF HEMATITE-ILLITE AND CHAMOSITE OOIDS FROM THE SILURIAN RED MOUNTAIN FORMATION (ALABAMA); A SOLUTION TO THE OOIDAL IRONSTONE ENIGMA

It has been proposed (Bhattacharyya, 1983) that the phyllosilicate berthierine (and its later diagenetic equivalent, chamosite), characteristic of ooids in Phanerozoic ironstones, is formed by the chemical reaction of iron oxide with kaolinite. However, while similarities in structure and Al₂O₃/SiO₂ ratios support this hypothesis, intermediates with compositions between berthierine-chamosite and kaolinite have not been reported (Maynard, 1986). During the examination of ironstones from the Silurian Red Mountain Formation, hematite-illite ooids were identified on the basis of SEM-EDS and x-ray diffraction. The presence of illite, rather than kaolinite, is indicated by analyses with K₂O and a relatively low Al₂O₃/ SiO₂ ratio (0.67 compared to 0.85 in kaolinite). While chamosite ooids occur in offshore shelf facies within the Red Mountain Formation, ooids in shoreface facies are dominated by hematite with small amounts of illite especially in the inner cortex. This suggests that iron oxide and clay mineral reacted to produce berthierine-chamosite in postoxic environments but failed to react under oxic conditions. It also indicates the possibility of formation from detrital clays other than kaolinite. Apparently, K₂O was lost and the Al₂O₃/SiO₂ ratio altered during the neoformation of berthierine-chamosite from detrital clay and iron oxide. A similar process is envisioned for the formation of glauconite and odinite in modern shelf sediments (Odin, 1988) and also for the precipitation of authigenic clays in microbial biofilms (Konhauser et al. 1993, 1998). Colloidal iron was most likely transported together with clay and microbial consortia and precipitated as impure ferric (oxyhydr)oxide in shoreface environments, but, despite an oxic water column, converted to ferrous phyllosilicate by dissimilatory reduction in postoxic microenvironments within ooids on the shelf. This explains the apparent contradiction in the occurrence of ferrous iron phyllosilicates as polished ooids in agitated oxic environments.