Nouveau concept sur les réactions de passage : kaolinite-mullite
Authors: Brindley George William , Nakahira M. 
Published: January 2018

The thermal reactions taking place when kaolinite is heated to high temperatures, to 1400 °C or higher, have been the subject of investigation for almost a century. One recalls the early work of Le Chatelier (1887), De l'Action de la Chaleur sur les Argiles, and of Mellor and Holdcroft (1911), The Chemical Constitution of the Kaolinite Molecule. Among the difficulties which arise in studying these reactions, the following may be mentioned :

(i) The poor crystallinity of metakaolin and the long uncertainty whether this is a mixture of oxides or an alumino-silicate.

(ii) The uncertainty as to the role of the spinel-type phase which forms at about 925-950°C and which has been regarded as γ-Al2O3 since 1928.

(iii) The structural relation, if any, of mullite to the parent kaolinite.

In a series of papers to be published in the Journal of the American Ceramic Society, the present authors discuss a new concept of this reaction series based on an X-ray single crystal study of kaolinite and its reaction products. This approach to the problem shows conclusively that a series of ordered reactions takes place from kaolinite, through metakaolin, to a cubic spinel-type phase (the socalled γ-Al2O3) and eventually to mullite. The key to the understanding of this sequence is the character of the orientation of the spinel phase. The close-packed planes of the spinel lie parallel to the oxygen and hydroxyl sheets of kaolinite, and the close-packed direction, the cube-face diagonal of the spinel, lies parallel to the baxis of kaolinite. Thus crystallographic order is preserved to a considerable extent in these transformations. The most reasonable interpretation is that the tetrahedral sites in the spinel are occupied by Si atoms and the octahedral by A atoms.

The probable composition of the spinel is Si3Al4O12 which, at first sight, seems considerably different from the usual spinel-type formula AB2O4. A comparison between these formulae and also that of γ-Al2O3 is best made on the basis of 32 oxygen atoms, which leads to the following :

normal spinel : A8B16O32

proposed Si-Al spinel : Si8 Al2O3 Al2O3 Al10 2/3 O32

γ~Al2O3 : 2 2/3 Al21 Al2O3 OAl2O3

One sees that 8 Si atoms occupy the 8A tetrahedral positions and that 5 1/3 vacancies occur among the 16B octahedral sites. By comparison, γ~Al2O3 has 2 2/ 3 vacancies which

may be distributed over all cation sites.

This concept makes it easy to understand the structural continuity from kaolinite, through metakaolin, to the spinel-type phase. The proportion of silica discarded at this stage of the reaction is small in accordance with the observation that little cristobalite is formed at this temperature. Ideally the equation can be written :

Si4Al4O14 -- Si3Al4O12 + SiO2

The final stage of the reaction involves further discard of silica and the formation of mullite, and additional cristobalite. The ordered formation of mullite, which is a chain-type silicate, proceeds from the chains of Al-octahedra already existing in the spinel-type structure.

There are many points of detail to be considered in relation to this new concept of the kaolinite reaction series, and these will be discussed by the authors in the forthcoming publications. Here it must suffice to emphasise that this new concept is based on a crystal-lographic interpretation of the reactions. Single crystal studies alone will establish that solid state reactions take place in an orderly sequence, with rearrangement of cations within an oxygen framework that remains largely (not wholly) continuous. One sees that 8 Si atoms occupy the 8A tetrahedral positions and that 5 1/3 vacancies occur among the 16B octahedral sites. By comparison, γ~Al2O3 has 2 2/ 3 vacancies which

may be distributed over all cation sites.

This concept makes it easy to understand the structural continuity from kaolinite, through metakaolin, to the spinel-type phase. The proportion of silica discarded at this stage of the reaction is small in accordance with the observation that little cristobalite is formed at this temperature. Ideally the equation can be written :

Si4Al4O14 -- Si3Al4O12 + SiO2

The final stage of the reaction involves further discard of silica and the formation of mullite, and additional cristobalite. The ordered formation of mullite, which is a chain-type silicate, proceeds from the chains of Al-octahedra already existing in the spinel-type structure.

There are many points of detail to be considered in relation to this new concept of the kaolinite reaction series, and these will be discussed by the authors in the forthcoming publications. Here it must suffice to emphasise that this new concept is based on a crystal-lographic interpretation of the reactions. Single crystal studies alone will establish that solid state reactions take place in an orderly sequence, with rearrangement of cations within an oxygen framework that remains largely (not wholly) continuous.