A COMPARISON BY SEM OF TEXTURE AND COMPOSITION IN HEMATITE AND CHAMOSITE OOIDS FROM THE BIRMINGHAM IRONSTONES OF ALABAMA, U.S.A

The Birmingham ironstones of the Silurian Red Mountain Formation are tidal shoreface facies dominated by hematite ooids and skeletal debris coated and replaced by iron oxide. However, chamosite ooids also occur locally in offshore limestone facies. The present study was undertaken to compare hematite and chamosite ooids under SEM in order to determine the origin (synsedimentary or diagenetic) and relationship between these two iron minerals. Both chamosite and hematite ooids are similar in size and flaxseed shape, and under SEM both reveal a delicately laminated cortex (not obvious in thin sections of hematite ooids). However, both chemistry and mineralogy differ. Hematite cortices are invariably contaminated by Si, Al and K suggesting a mixture of hematite and illite. In several cases the outer cortex is dominated by hematite, while illite increases towards the nucleus. Chamosite ooids, on the other hand, lack K and are apparently pure chamosite (Fe, Mg, Si, Al). In most cases fluorapatite is also present (Ca, P, F), but is difficult to distinguish in thin section. The presence of illite is unexpected. It is probably detrital and supports the conclusion that iron was transported from the hinterland as a colloidal complex with clay, in this case illite. The absence of K in chamosite ooids is probably the result of leaching, and makes it more likely that chamosite was derived by reduction from iron oxide and clay than the reverse. The perfection of the lamination in the cortex of both chamosite and hematite ooids suggests that mineral precipitation was interrupted by episodes of abrasion and polishing, and thus, that both minerals are original precipitates rather than formed by replacement, one by another. If the flaxseed shape is indicative of microbial precipitation, as many workers suspect, the mineral species may depend on the metabolism of the microbes.