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

• Published in: Clays and clay minerals 1997-06, Vol.45 (3), p.339-352
• Main Authors: RYAN, P. C, REYNOLDS, R. C
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Mineralogy; Petrology of sedimentary rocks except quaternary rocks; Sedimentary rocks; Silicates
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1346/ccmn.1997.0450305

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 Al. XRD-determined compositions appear to be erroneous and actually reflect progressive changes in Sp/Ch unit-cell dimensions caused by polytype transformations of I bb layers to I aa layers in a mixed-layer I bb/ I aa 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 I bb layers to I aa 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 Al 3+ that prohibit inclusion of the highly charged Ti 4+ in the octahedral sheet. Anatase (TiO 2 ) 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). Odinite, an Fe-rich 7-Å 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.