Ceramic and Glass Materials

44 R.C. Bradt

the coordination of the Al3+ cations that characterizes the structural differences. In
andalusite, which is also orthorhombic in structure, the AlO 6 octahedra chains are
linked by SiO 4 tetrahedra and AlO 5 polyhedra alternate within the structure.
Proceeding to kyanite, the chains are linked by SiO 4 tetrahedra and AlO 6 octahedra. It
is evident that the major structural difference for the three sillimanite minerals is the
Al3+ cation coordination that forms the linkages between the double AlO 6 octahedra
chains. In all three minerals, the Si4+ cations are always tetrahedrally coordinated with
oxygens, while half of the Al3+ cations are always octahedrally coordinated with oxygens.
It is the coordination of the other half of the Al3+ cations that changes in these struc-
tures. In sillimanite, the other half of the Al3+ cations are in fourfold or tetrahedral
coordination, but, in anadalusite, they are in fivefold coordination. In kyanite, they are
in sixfold or octahedral coordination. This critical change of the Al3+ cation coordination
in the three minerals occurs in those polyhedra that crosslink the double chains that
are formed by the edge sharing of the AlO 6 octahedra. Such distinctive structural dif-
ferences can be considered to be a manifestation of the high pressures involved. The
cation coordination numbers are presented as the superscript Roman numerals on the
mineral chemical formulae in Table 1.
The structure of mullite is similar to that of the sillimanites, consistent with the fact
that they decompose to form mullite at high temperatures and 1 atm pressure. It has
been suggested that the double AlO 6 octahedral chain structure is preserved during the
decomposition. The mullite structure is, however, somewhat complicated by its exten-
sive stability over a wide range of stoichiometries. The composition of mullite can be
expressed as

Al Al() 12 +− −xx xSi 12 O 5 (1)

where the value of x may vary from ∼0.08 < x < ∼0.29. It is evident that the common
3:2 mullite is not the only stoichiometry. The 2:1 mullite structure is common in
fusion cast mullites. In mullite, the AlO 6 octahedra also form double chains parallel
to the c-axis of the orthorhombic structure and are also cross-linked by alumina and
silica tetrahedra. This is one reason why mullite is often considered along with the
sillimanite minerals in the literature. These structural aspects of mullite, along with
those of the three sillimanite minerals, are summarized in Table 1, which allows for
their direct comparisons on the basis of several different physical properties.

3 Decomposition of the Sillimanite Minerals

As the sillimanite minerals are geologically formed at high pressures, they decompose

or undergo a structural decomposition when heated to elevated temperatures in air at

1 atm pressure [7–10]. The decomposition reaction can be written as

3332 Al O SiO 23 ⋅= 2 Al SiO 2 5 → ⋅ +Al O 23 SiO 2 SiO 2 (2)

where the 3:2 stoichiometric mullite and silica are the decomposition products.
The previously mentioned 2:1 mullite does not form during the decomposition of the
sillimanites. The form of the silica varies for the three polymorphs. For kyanite, the