Letter | Published:

Exothermic Reaction of Metakaolin between 950° and 1,000° C.


RECENTLY, Wiegmann and Horte1 have discussed the exothermic reaction of metakaolin and concluded that it is due to the formation of sillimanite. Opinion on this question has long been divided. Some2 ascribe the reaction to the crystallization of γ-Al2O3, whereas others3 ascribe it to the formation of mullite by rearrangement of the metakaolin lattice, yielding a residue of γ-Al2O3 and amorphous silica. The claim that sillimanite rather than mullite forms at the exotherm re-introduces a hypothesis that has long gone out of favour4,5. The evidence put forward by Wiegmann and Horte is difficult to dispute but nevertheless the results of our work do not agree with theirs.

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  1. 1

    Wiegmann, J., and Horte, C. H., Z. anorg. Chem., 286, 268 (1956). Horte, C. H., and Wiegmann, J., Naturwiss., 43, 9 (1956). Wiegmann, J., and Horte, C. H., Silikat technik, 7, 417 (1956).

  2. 2

    Insley, H., and Ewell, R. H., J. Res. Nat. Bur. Standards, 14, 615 (1935). Richardson, H. M., “X-Ray Identification and Crystal Structures of Clay Minerals”, Chap. 3, 76–85, ed.: Brindley, G. W. (Mineralogical Society, London, 1951). Colegrave, E. B., and Rigby, G. R., Trans. Brit. Ceram. Soc., 51, 355 (1952).

  3. 3

    Comeforo, J. E., Fischer, R. B., and Bradley, W. F., J. Amer. Ceram. Soc., 31, 254 (1948). Johns, W. D., Miner. Mag., 30, 186 (1953). Lebedev, J. W., C.R. Acad. Sci., U.R.S.S., 89, 335 (1953). Glass, H. D., Amer. Min., 39, 193 (1954). Roy, R., Roy, D. M., and Francis, E. E., J. Amer. Ceram. Soc., 38, 198 (1955). Vaughan, F., Clay Min. Bull., 2, 265 (1955).

  4. 4

    Posjnak, E., and Greig, J. W., J. Amer. Ceram. Soc., 16, 569 (1933).

  5. 5

    Letort, Y., Internat. Ceramic Cong., 19 (1952).

  6. 6

    De Keyser, W., Trans. Brit. Ceram. Soc., 50, 349 (1951).

  7. 7

    Scholze, H., Ber. deutsch. keram. Ges., 32, 381 (1955).

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