Abstract
Two different types of mullite could be determined in differently coloured fragments of South American bauxites: − Mullites, occuring in light bauxite fragments with low impurity contents (Σ wt % (Fe 2O 3 + TiO 2) ⪅ 2.5), and cell dimensions close to that of 3 2 mullite, the b parameter being slightly shorter. − Mullites, occurring in grey to dark grey, and in brown bauxite fragments, with high impurity contents (Σ wt % (Fe 2O 3 + TiO 2) ⪅ 4,0) and a and c cell dimensions close to those of 2 1 mullite. The b constant of these mullites is slightly expanded with respect to impurity free mullites. Increasing impurity contents in mullite are linearly correlated with a b expansion. The expansion was structurally explained with a substitution of Al 3+ by Fe 3+ and Ti 4+ at octahedral lattice sites, which causes stretching of the AlO 6 octahedra along to the elastic Al 1D D bond in mullites which is 30° to either side of b. The substitution-produced expansion is superimposed by an expansion due to the change of the structural state of mullite from the 3 2 to the 2 1 type. The greatest variation in any cell parameter with the change of the structural state is the a constant.
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