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

Abstract

Abstract--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.

Pre-requisites, processes, and prediction of chlorite grain coatings in petroleum reservoirs: A review of subsurface examples

“…A similar relationship was noted in Tuscaloosa Formation, Gulf Coast, USA (Ryan and Reynolds, 1997). With increasing burial, mixed-layer chloriteeserpentine became more chloritic.…”

Diagenesis and clay mineral formation at Gale Crater, Mars

“…[] with XRD data. These phases are relevant to the low temperature, diagenetic type of environment we are considering [e.g., de Caritat et al ., ], although they might, however, form as mixed layer clays [ Ryan and Reynolds , ]. All phyllosilicates in the model are added to derive the clay chemistry with a weighted average composition.…”

“…Thus, in our runs, we take a nontronite + chlorite assemblage as a chemical analogue to the clay identified by Vaniman et al [2014] with XRD data. These phases are relevant to the low temperature, diagenetic type of environment we are considering [e.g., de Caritat et al, 1993], although they might, however, form as mixed layer clays [Ryan and Reynolds, 1997].…”

Chlorite-White Mica Pairs’ Composition as a Micro-Chemical Guide to Fingerprint Massive Sulfide Deposits of the Bathurst Mining Camp, Canada

“…In the case of Ti, biotite and white mica host the bulk Ti budget of the host rocks in the BMC (see [22] and this study), and Ti-bearing oxides are expected to form during the alteration of biotite to chlorite (see [22]). The lower budget of Ti of chlorite may reflect the high levels of octahedral Al unique to the chlorite structure [68]. The titanium content of chlorite generally increases trend with proximity to the ore horizon, a trend related to higher temperature up-flow zone (see Supplementary Table S6).…”