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Department of Geology, Beaumont Building, The University, Brookhill, Sheffield, S3 7HF, UK
Analytical transmission electron microscopy (ATEM) has been used by the author to obtain textural, crystallographic and quantitative chemical information from examples of berthierine-rich and chamosite-rich oolitic ironstones. Individual phases were often found to be too small to have been readily analysed by other techniques. Berthierine laths (of the order of 20 x 500 to 1000 nm) and goethite grains (of the order of 25 nm in diameter) were found to exhibit unorientated microtextures, which were independent of both depositional and oolitic fabrics. Chemical analyses of berthierines revealed fairly uniform compositions regardless of textural association. Chemical data from the chamosite-rich samples appeared analogous to that of the berthierine-rich material, although average crystal sizes were found to be much larger (chamosite laths varied from 0.1 x 1.5 µm to 0.5 x 2 mµ). The compositional data suggest that the chamosite-rich oolitic ironstones are simply the late stage diagenetic (or low grade metamorphic) recrystallized equivalents of berthierine-rich precursors. No direct evidence indicative of the mode of origin for berthierine and goethite ooliths was found. It is suggested that ancient ironstone ooliths were probably formed via similar mechanisms to those which generate modern and ancient carbonate ooliths, although obviously the original sediment and interstitial fluids would have been very different.
