THE CHEMICAL-COMPOSITION OF SERPENTINE CHLORITE IN THE TUSCALOOSA FORMATION, UNITED-STATES GULF-COAST - EDX VS XRD DETERMINATIONS, IMPLICATIONS FOR MINERALOGIC REACTIONS AND THE ORIGIN OF ANATASE/

The chemical composition of mixed-layer serpentine/chlorite (Sp/Ch) in Tuscaloosa Formation sandstone was analyzed by energy dispersive X-ra y spectroscopy (EDX) in the scanning electron miscroscope (SEM) and by X-ray diffraction (XRD). EDX results indicate little depth-controlled Variation in composition, whereas XRD results suggest distinct decrea ses in octahedral Fe and tetrahedral Al. XRD-determined compositions a ppear to be erroneous and actually reflect progressive changes in Sp/C h unit-cell dimensions caused by polytype transformations of Ibb layer s to Iaa layers in a mixed-layer Ibb/Iaa polytype. The relative lack o f variation in Sp/Ch composition, especially when compared to other st udies of chlorite minerals over similar temperature ranges, is attribu ted to a reaction mechanism whereby mineralogic transformations (serpe ntine layers to chlorite layers and Ibb layers to Iaa layers) occur on a layer-by-layer basis within coherent crystallites, rather than by d issolution-precipitation crystal growth. The lack of titanium in chlor ite minerals is attributed to high levels of octahedral Al3+ that proh ibit inclusion of the highly charged Ti4+ in the octahedral sheet. Ana tase (TiO2) in the Tuscaloosa Formation apparently formed when Ti was Liberated during crystallization of Sp/Ch following the breakdown of a Ti-bearing precursor (detrital ultramafic clasts and/or odinite). Odi nite, an Fe-rich 7 Angstrom phyllosilicate that forms in some shallow marine sands, apparently existed as a short-lived, poorly crystallized intermediary between dissolution of the ultramafic clasts and formati on of Sp/Ch.