Cation distribution, sheet thickness, and O-OH space in trioctahedral chlorites - an X-ray and infrared study.

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.

Section: High Charges Of Ti 4+ and A13+mentioning

confidence: 99%

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

Ryan

Reynolds

1997

“…Trioctahedral chlorites have been poorly studied by spectroscopic techniques. Most of the available data concern infrared (IR) absorption spectroscopy, and some correlations between vibrational bands and chemical composition have been made (Tuddenham and Lyon, 1959;Stubican and Roy, 1961;Hayashi and Oinuma, 1965;Oinuma and Hayashi, 1968;Farmer, 1974;Shirozu and Nomoi, 1972;Shirozu et al, 1975;Shirozu, 1980Shirozu, , 1985. Most of these studies, however, are not complete and fail to take into account all solid solutions.…”

Section: Introductionmentioning

confidence: 99%

Structure-Composition Relationships in Trioctahedral Chlorites: A Vibrational Spectroscopy Study

Prieto

Dubessy

Cathelineau

1991

Abstract--Raman and Fourier-transformed infrared spectra of natural trioctahedral chlorites of polytype IIb were obtained for a series of samples characterized by distinct Fe/(Fe + Mg) and Si/Al ratios ranging, respectively, from 0.04 to 0.94 and from 5.18 to 1.86. All samples were characterized by X-ray powder diffraction, and quantitative electron microprobe analysis. In the 3683-3610-cm -~ spectral range, the wave number of the OH-stretching band from the 2: l layer (band I) decreased with an increase of iron content at constant Al(IV) content. The more intense bands II and III at about 3600 cm l and 3500 cm -1, were assigned to hydroxyl groups involved in hydrogen bonds: (SiSi)O... HO, with the hydrogen bonds being roughly perpendicular to the basal plane, and (SiA1)O... HO, respectively. At higher tetrahedral A1 and octahedral Fe contents, spectra exhibited OH-bands II and III, respectively, at a lower frequency. Band III intensity increased and band II was enlarged for chlorites displaying higher AI(IV) contents.In the 1300-1350-cm l spectral range, most infrared spectra displayed intense bands at 1090, 1050, 990, and 960 cm ~, which were assigned to T-O stretching of symmetry species A1 and El. The second type of bands observed both in Raman and infrared spectra were at about 650-800 cm ~; they were assigned to OH vibrations and were strongly dependent on the composition of the interlayer octahedral sheet, especially on the Fe content.

“…The first one is assigned to hydroxyls of the 2:1 layer and the other two, to hydroxyls of the interlayer sheet which are hydrogen bonded to oxygens of the tetrahedral sheet (Shirozu 1980(Shirozu , 1985. In the 1100-400 cm ~ region the IR spectra can be divided into three frequency domains: 1) in the 1000 cm -~ range absorption peaks are assigned to lattice vibrations; 2) in the 850-4550 cm ~ range, the vibrations at 827, 750 and 670 cm -~ are assigned to tetrahedral AI-O Table 1.…”

Section: Introductionmentioning

confidence: 99%

Development of Microporosity in Clinochlore Upon Heating

Villièras

Yvon

Cases

et al. 1994

Abstract--The "modified chlorite structure" forms by the dehydroxylation of the interlayer octahedral sheet of magnesian chlorite at around 500~ and results in a structure with a basal spacing near 27/~ (Brindley and Chang 1974). This process involves drastic textural modifications as indicated by gas adsorption experiments which reveal the formation of structural micropores. Infrared spectroscopy as well as thermogravimetry and mass spectrometric analysis show that these micropores are filled with molecular atmospheric water, carbon dioxide, nitrogen, argon and hydrocarbons which condense once the samples cool down. A high temperature treatment is needed in order to release the different phases. A heterogeneous dehydroxylation mechanism is proposed in which micropores are formed in donor regions and magnesium and oxygen are concentrated in acceptor regions. This leads to a 27/~ structure with micropore zones and enriched interlayer oxide zones alternating along the z-axis of the mineral.