MODELLING OF MULLITE SOLID-SOLUTIONS IN THE SYSTEM Al2O3-SiO2

A model for the mullite formation from kaolinites and the solid-solution range of mullite in the Al2O3-SiO2 system has been evaluated. This rather complicated model implies that both octahedral and tetrahedral coordinated Al3+-ions as well as tetrahedral Si4+-ions participate together with O2-ions and vacant oxygen positions during the rearrangements in the structure during the sintering process at high temperatures.


Introduction
It is well known that mullite, 3Al 2 O 3 •2SiO 2 , is the only compound formed between the compounds in the binary system Al 2 O 3 -SiO 2 at ordinary pressures (1 bar) and high temperatures.For many years there has been a continuous discussion whether mullite melts congruently or incongruently.Today it seems to be accepted that mullite melts incongruently at ~ 1828 o C according to the equation: (1) The crystal structure of mullite has been interpreted for a long time as a modified defect structure of sillimanite, in which the mullite stoichiometry is achieved by substituting Si 4+ -ions with Al 3+ -ions in the tetrahedral sites of the alternating aluminium and silicon columns.
Holm and Kleppa [1] for instance suggested in 1966 that the mullite structure could be derived from the structure of sillimanite, Al VI [Al IV Si]O 5 , by distributing the tetrahedrally coordinated aluminium and silicon atoms over their formerly ordered positions as given by : This model was also accepted and used by Aksay et al [2] in their work from 1991.They suggested the following scheme for the substitution in the lattice and stated that in their model for the solid-solution in the system the octahedral AlO 6 clusters do not change during the rearrangements in the structure.They expressed the following defect structure model for the mullite composition: (3) However this model has recently been questioned by Holm [3,4].His calculations show that the unit cell of sillimanite is not comparable in size with the unit cell of mullite.
From the formula given for mullite, molecular weight of 159.77 g mol -1 , a density 3.17 g cm -3 and cell volume 167.35 Å 3 , the Z value can be calculated: As can be seen from Table 1, both structures are orthorhombic, but while the unit cell of sillimanite contains 4[Al 2 SiO 5 ] units, the unit cell of mullite In a paper by Holm [4] on the kaolinites-mullite transformation in the Al 2 O 3 -SiO 2 system, gels of synthetic kaolinites formed from mixtures of very pure metalorganic compounds of aluminium and silicon and with seven different Al/Si ratios, were investigated by DTA/TG/DTG up to 1200 o C. The enthalpies associated with the exothermic reaction occurring at 980 o , were determined and plotted as a function of composition.The enthalpies obtained reached a maximum at a composition corresponding to 2Al 2 O 3 ×3 SiO 2 .The result was discussed with reference to the well known work by Brindley and Nakahira [5] from 1959 on the kaolinite-mullite reaction series.Good correlation between the experimental enthalpies and the enthalpies of crystallization of alumina and silica taken from the literature were obtained.Models for distribution of the Al-atoms between the octahedral and the tetrahedral positions in the mullite structure were discussed, and models for the incongruent melting of both 3:2 mullite and 2:1 mullite were also evaluated.

Results and Discussion
On the basis of the results referred in the introduction, the mullitization process and the formation of primary mullite will be discussed.A model for units.
the solid-solution formation in the system and the extent of the solid-solution range will also be discussed.
a) Al 4 Si 3 O 12 ("the spinel phase") The so-called "spinel phase" Al 4 Si 3 O 12 formed from kaolinite at 980 o C will be the logical starting point for a discussion of the mullitization process.By further heating above 1000 o C silica will be discarded from the spinel structure according to the equation: (5) and the Al 4 Si 2 O 10 -unit is formed.It is important to notice that this unit will be the basic unit for the formation of the different mullites in the Al 2 O 3 -SiO 2 system during futher heating.

b) Al 4 Si 2 O 10 (1:1 composition)
As already shown by Holm [4] the Al 4 Si 2 O 10 -unit can be considered to consist of two separate units given by: By further heating the following reaction will take place (7) The Al-ions outside the spinel phase will during the sintering process enter the lattice in octahedral positions while the Al 3+ -and Si 4+ -ions in the spinel phase will remain in tetrahedral positions.
Al 2 O 3 will start as follows: (8) which again will react during the sintring process at higher temperatures ( T> 1400°C) to the final 3:2 mullite, as has been shown by Holm [4].
A corresponding scheme can be given for the formation of the so-called 2:1 mullite from the following units ( 9) also here with a final distribution of the Al-ions for the 2:1 mullite in accordance with the distribution given in the paper by Holm [4].

e) Solid solution model for mullite
In the evaluation of a model for the solid solution between the three members in the system: one has to look at the distribution of the Al-ions between octahedral and tetrahedral positions as has been done in the following three schemes for their unit cells: The following solid-solution model can now be evaluated on the basis of the distributions given above: (10) This model implies that the following substitutions occur between the members in the composition range 50 -67 mol% Al 2 O 3 in the system: This is different from the model or scheme given in Eq. (3).
For the 3:2 mullite composition this model gives the following distribution: compared to one given in this work: 2 give compositions, cell dimensions as well as calculated densities for seven members belonging to the solid solution series in the composition range 50-100 mol% Al 2 O 3 .The distributions on octahedral and tetrahedral sites are calculated from Eq. (10).The cell dimensions are taken from the works of Cameron [6,7] for four of the members.For the 7:4 and 2:1 compositions the data are own determinations (see appendix).
The densities are plotted as a function of n vacancy in Fig. 1.As can be seen the calculated densities fit nicely to a straight line.The density obtained by extrapolation to n vacancy = 1 is 2.93 g/cm 3 .
dmeas.g/cm 3   1: Al O The end member, corresponds to a mullite-type silica free aluminate, and has been discussed in the literature before by Foster [8] in 1959 and by Perrotta and Young [9] in 1974.Their data are given in Table 3 together with our own calculated d-values based on a tetragonal cell with lattice constants a = b = 7.69 Å and c = 2.905 Å, again taken from the work by Cameron [6].
*This work, extrapolated cell dimensions from Cameron [6] Table 3. X-Ray diffraction data for mullite-type aluminate (m-Al The results from the X-ray investigation are given in Table 4.

Table 1 :
Cell parameters, volumes and densities for mullite and sillimanite

Table 2 .
Compositions and cell dimensions of compounds in the solid solution series in the Al2 O 3 -SiO 2 system