Interpretation of the Kaolinite‐Mullite Reaction Sequence from Infrared Absorption Spectra

Abstract

into a 16-part hyperfine structure, indicating either a low concentration of V" ions and/or their existence in an unsymmetrical field. The signal for V4+ in K-ALO, consisted of a single peak, indicating a higher concentration of V" or its existence in a symmetrical field.In view of the atmospheres used in the preparations and the concentrations of V determined by analysis, it seems likely that concentration of V" is responsible for the difference between these spectra. The role of V4+ in orthorhombic S(3)-ALO, was indicated by further study of conditions favoring its formation. In air it was obtained from S(2) between 1000" and llOO°C, as described, but a sample of y-ALO, containing 10 wt% V fired at 750°C in 10%-H2-in-N1 gas and then at 800°C in Or yielded S(3)-AI20, directly.The structure of K-ALO, was studied by Brindley and Choe,' who reported the existence of two modifications obtained by heating fine-grained gibbsite. Saalfeld' observed only one type, corresponding to the form rather less frequently observed by Brindley and Choe, in the electron microscope. The present K-ALO:, gave an X-ray powder diffraction pattern which could be indexed on the larger cell given by Brindley and Choe (Table I ) and contained several lines which could not be indexed on the smaller cell found by Saalfeld, i.e. the 211, 207, 430, and 208 reflections. Samples of K-ALO~ were also prepared from a very pure fine-grained gibbsite (Na,O content 4 ppm) by the fluxing action of 5% V20, in 0, or air at 1100°C for 1 h. The relative intensities of the X-ray powder reflections were the same for this product as for that obtained from y-ALO, as described. In addition, no change in the relative intensities, e.g. of the 208 and 604 planes in particular, was observed for further samples prepared at 900" to 1080°C in the presence of V4+. No evidence was obtained, therefore, for the existence of ~-Al,0, with the smaller cell, which was present in the poorly crystalline material obtained from gibbsite. However, electron diffraction examination of these samples revealed some patterns which could be indexed on both possible a,, values, whereas others could be indexed only on the larger value. It cannot be concluded, however, that a mixture was present.Although the role of V,O, as a flux, with an effect depending on amount and temperature, is fairly straightforward, the role of the V4' ion is not. Its action is apparently required in the early stages of firing if the poorly crystalline y-Al,O,, is to be "directed" into the K-form. which could be obtained from Alumina as a Ceramic Material. Compiled and edited by W. H. Gitzen. The American Ceramic Society, Inc., Columbus, OH, 1970. 2 P. H. Rookshy; pp. 354-92 in X-Ray Identification and Crystal Structures of Clav Minerals. Edited bv G. Brown. Mineraloeical Society, London, 1961. minum Hydroxide Gels,"

The Chemical Form of Vanadium (IV) in Kaolinite

“…Metakaolinite, an intermediate phase in the kaolinite-mullite reaction sequence, occurs in the temperature range between 550 ~ and 1000~ (Lombardi et al, 1987). In the metakaolinite structure, which exhibits a high concentration of lattice defects and no welldefined long-range order, AI(III) is in octahedral as well as tetrahedral coordination (Percival et al, 1974;Leonard, 1977). Computer simulations by MacKenzie et al (1985) show that the formation of metakaolinite leads to an additional, distorted, fourfold coordination for AI(III).…”

Structural Characterisation of Kaolinite:NaCl Intercalate and its Derivatives

“…The A1-O-H bending modes at 910 and 940 cm-t disappear. A strong band at ~ 700 cm -t is enhanced unlike in metakaolinite where a similar band emerges at ~800 cm -L (Percival et al, 1974;Rocha and Klinowski, 1990). There appears to be little structural change following rinsing (Figure 7d).…”

Changes in Structure, Morphology, Porosity, and Surface Activity of Mesoporous Halloysite Nanotubes Under Heating

“…The thermal dehydroxylation and transformation behavior of kaolinite is one of the most thoroughly investigated reactions (Rocha and Klinowski, 1990) because of its importance for ceramic processing. The intermediate states and the equilibrium end point of the kaoliniteÀmullite reaction have been well defined by various methods including magic-angle spinning nuclear magnetic resonance (MAS NMR) MacKenzie et al, 1985;Rocha and Klinowski, 1990;Massiot et al, 1995), transmission electron microscopy (TEM) (Bergaya et al, 1996;Lee et al, 1999), infrared spectroscopy (Percival et al, 1974;Vassallo et al, 1992;Frost and Vassallo, 1996), electron paramagnetic resonance (Djemai et al, 2001), X-ray diffraction (XRD), and thermal analysis (Dion et al, 1998; Kristóf et al, 1998).…”