Diagenetic Illite-Chlorite Assemblages in Arenites. II. Thermodynamic Relations

Aagaard, Per; Jahren, Jens

doi:10.1346/ccmn.1992.0400508

Cited by 18 publications

(8 citation statements)

References 23 publications

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“…The amount of corrensite interlayered with chlorite may depend on the rate of nucleation and crystal growth, therefore indirectly on the degree of supersaturation in solution as well as temperature. Aagaard and Jahren (1992) related an increase in A1 content of authigenic chlorite in sandstones to a Tschermak's type exchange reaction with coexisting illite under increasing temperature. The data are EDS-TEM analyses from Jahren and Aagaard (1992) that show a broad scatter in the A1 contents of illite and chlorite; the chlorite analyses also have a substantial excess of AI v1 over AI TM, indicative of corrensite interlayers or of substantial dioctahedral substitution.…”

Section: Chlorite and Illite Crystallinitymentioning

confidence: 96%

Clay Mineral Thermometry—A Critical Perspective

Essene

Peacor

1995

Clays and clay miner.

229

129

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Abstract--Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compositional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error.Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers' rules and indicate lack of equilibrium.Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral thermometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

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Section: Chlorite and Illite Crystallinitymentioning

confidence: 96%

Clay Mineral Thermometry—A Critical Perspective

Essene

Peacor

1995

Clays and clay miner.

229

129

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“…6, corresponds to an activity of muscovite equal to 0.6 (cf. Aagaard & Jahren, 1993) Reaction fluids from experimental runs both at 250 and 2008C are plotted onto the aqueous activity diagram for 2508C and 43 bar. This is done simply for illustration.…”

Section: Mineral Water Relationsmentioning

confidence: 99%

Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences

et al. 2000

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Grain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

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“…Chemical potential diagram and diffusion path.--Helgeson et al (31)(32)(33) have extensively investigated on kinetic phenomena between minerals and aqueous solutions under the local equilibrium approximation. When compared with such systems, diffusion in solid states strongly depends on the crystal structures, the availability of vacancies and interstitial sites, grain size, grain boundaries, and so on.…”

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confidence: 99%

Thermodynamic Analysis of Reaction Profiles Between LaMO3 ( M = Ni , Co , Mn ) and ZrO2

Yokokawa

Sakai

Kawada

et al. 1991

J. Electrochem. Soc.

150

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Chemical reactions of LaMO3(M=Ni, Co, Mn) and ZrO2 have been analyzed using chemical potential diagrams for the La-Zr-M-O systems under a condition of P(O2)= 1 bar. A log{a(M)/a(Zr)} vs. log{a(La)/a(Zr)} plot is appropriate for representing diffusion paths, because the layered arrangement of the reaction zone corresponds well to the geometrical configuration of stability polygons of reactants (LaMO~ and ZrO2) and products (La2Zr2OT, MO, La2ZrMO6, etc.). A complicated arrangement including a ternary perovskite phase, La2ZrNiO6, observed in the La2NiO~SZ couple can be represented by ABSTRACT Process control strategies have been developed for reactive ion etching of silicon and silicon dioxide in CF4/O2 and CF4/H2 plasmas. Four measured variables, four manipulated variables, and seven performance variables were considered for both gas mixtures. Relative gain array analysis and singular value decomposition were used to select manipulated/ process variable control loop pairings for feedback control and to evaluate potential difficulties in control system perform-

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Diagenetic Illite-Chlorite Assemblages in Arenites. II. Thermodynamic Relations

Cited by 18 publications

References 23 publications

Clay Mineral Thermometry—A Critical Perspective

Clay Mineral Thermometry—A Critical Perspective

Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences

Thermodynamic Analysis of Reaction Profiles Between LaMO3 ( M = Ni , Co , Mn ) and ZrO2

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