Reactions a N D the Origin of a N a T a S E
نویسنده
چکیده
-The chemical composition of mixed-layer serpentine/chlorite (Sp/Ch) in Tuscaloosa Formation sandstone was analyzed by energy dispersive X-ray 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 decreases in octahedral Fe and tetrahedral A1. XRDdetermined compositions appear to be erroneous and actually reflect progressive changes in Sp/Ch unitcell dimensions caused by polytype transformations of Ibb layers to Iaa layers in a mixed-layer IbblIaa polytype. The relative lack of variation in Sp/Ch composition, especially when compared to other studies of chlorite minerals over similar temperature ranges, is attributed to a reaction mechanism whereby mineralogic transformations (serpentine layers to chlorite layers and Ibb layers to Iaa layers) occur on a layer-by-layer basis within coherent crystallites, rather than by dissolution-precipitation crystal growth. The lack of titanium in chlorite minerals is attributed to high levels of octahedral A13§ that prohibit inclusion of the highly charged Ti 4+ in the octahedral sheet. Anatase (TiO2) in the Tuscaloosa Formation apparently formed when Ti was liberated during crystallization of Sp/Ch following !he breakdown of a Ti-bearing precursor (detrital ultramafic clasts and/or odinite). Odinite, an Fe-rich 7-A phyllosilicate that forms in some shallow marine sands, apparently existed as a short-lived, poorly crystallized intermediary between dissolution of the ultramafic clasts and formation of Sp/Ch. Key Words---Anatase, Chemical Composition, Diagenesis, Sandstone, Serpentine/Chlorite, Tuscaloosa Formation.
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