Structural mechanism of thermal and compositional transformations in silicates

Donnay, Gabrielle; Wyart, Jean; Sabatier, Germain

doi:10.1524/zkri.1959.112.1-6.161

Cited by 58 publications

(10 citation statements)

References 2 publications

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“…The formation of metakaolinite involves a change in the coordination of the A1 cations from 6 to 4, and the oxygen atoms bridging two 4-coordinated aluminum atoms may provide the sites needed for hydrogen hopping. Similar mechanisms have been proposed to account for Si/A1 mobility in the solid state (Donnay et al 1959), and to explain the combined effect of pressure and hydrogen in the interdiffusion of tetrahedrally coordinated Si and A1 atoms in albite and microcline feldspars (Goldsmith and Jenkins 1985;Goldsmith 1987Goldsmith , 1988. Such a mechanism might well explain the differences in the reactivity observed between materials with different degrees of structural defects.…”

Section: Discussionmentioning

confidence: 60%

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

et al. 1995

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When dehydroxylation of kaolinite powder is carried out in the usual way, the linear relations anticipated for first-order kinetics and for the Arrhenius plot of log k versus 1/T are satisfied only very approximately. Factors relating to the form of the specimen, (shape, size, compaction, container, etc.) are shown to be very important. A method is developed for obtaining data for a specimen in the form of an infinitely thin disc. The first-order kinetic relation and the Arrhenius relation are then linear, and the latter gives an activation energy of 65 K cal./mol. The dehydroxylation process is shown by x-ray analysis to proceed crystal by crystal and this leads to an interpretation of the first-order kinetics. The x-ray method is used to study the distribution of reacted and unreacted material throughout a disc of material. Although isothermal conditions are employed, large differences are found between the interior and exterior of a partially dehydroxylated disc. These effects ~e attributed to the influence of a water vapor atmosphere within the heated disc. (1) INTRODUCTION The loss of weight when kaolinite is fired to temperatures exceeding about 450~ under normal atmospheric conditions is commonly ascribed to "dehy-dration" and the water involved in the reaction is designated "structural water." Neither term is correct, as the crystal lattice loses hydroxyl groups. The process is better described as "dehydroxylation" and it can be represented chemically by the equation OH+OH=H2OI' +O The mechanism is most probably one of proton migration so that if two protons momentarily find themselves associated with the same oxygen ion, there is a probability that a water molecule will be formed and will detach itself from the lattice. The present work was undertaken in the hope that a kinetic study of the process accompanied by x-ray examination would provide more detailed information about the process than is currently available. The crystal structure of kaolinite (Brindley and Robinson, 1946; Brindley and Nakahira, 1956) is so wet| known that no detailed description is required here. It suffices to recall that the four (OH) groups of the structural unit AI2Si2Os(OH)4 are located wholly in the octahedral sheet of the structure, three of them occupying positions on the outside of the layer structure and one within the structure. A previous study of the dehydroxylation of kaolinite 1 Contribution no. 56-35 from the College

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Section: Discussionmentioning

confidence: 60%

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

et al. 1995

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“…The value of the activation energy, in the range 25-60 kcal/mole, points to a chemical diffusion process involving the breaking of chemical bonds (Brindley and Nakahira 1957;Brindley et al 1967). By analogy with the interdiffusion processes of tetrahedrally coordinated Si and A1 atoms in framework structures (Donnay et al 1959;Goldsmith and Jenkins 1985;Goldsmith 1987Goldsmith , 1988, we suggested that hydrogen or hydroxyl diffusion is involved in the process, and that the 4-coordinated A1 ions present in metakaolinite may provide the sites for hopping.…”

Section: Introductionmentioning

confidence: 80%

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

et al. 1995

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Abstract. The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part.Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase.The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion.For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite.The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic geMike phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.

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“…Many previous authors (e.g. Donnay et al 1959;Martin 1974) considered water necessary to promote ordering in alumino-silicate al-bite. The present study suggests that the apparent absence of water in no way inhibits ordering in gallium albite.…”

Section: Order-disorder Transformationmentioning

confidence: 99%

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

Burns

Fleet

1990

Phys Chem Minerals

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Abstract.The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi308) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaA1Si3Os]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi308, grown in a Na2WO4 flux, was ordered by hydrothermal annealing below 820~ and dry annealing above 820 ~ C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890~ 970 ~ C. The gallium content of the Tao site increases continuously with decreasing temperature. The gallium contents of the Tam and T2m sites decrease smoothly with increasing ordering while the gallium content of the 7"2o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913_+3~ C and 937+3 ~ C. These abrupt changes correlate with changes in the gallium content of the T2o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGao.sAlo.sSi3Os) occurs in the temperature range 765 ~ C-850 ~ C, at a temperature intermediate to the transformation in albite (50% order at about 680 + 20 ~ C) and gallium albite.

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Structural mechanism of thermal and compositional transformations in silicates

Cited by 58 publications

References 2 publications

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

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