Structures of the Sillimanite Minerals
The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with O2- anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with O2- anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within AlO5 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite.
In sillimanite itself, where the Al3+ cations are in four – and sixfold coordination, double chains of aluminum oxide octahedra exist parallel to the c-axis of the orthorhombic structure. These chains are formed by the edge sharing of the AlO6 octahedra. Those chains are linked or connected by alternating AlO4 and SiO4 tetrahedra. Note the uniqueness of the presence of Al3+ cations in fourfold coordination, which is again a consequence of the mineral formation at high pressures. The coordination of the Al3+ cations in sillimanite is evenly divided. Half of the Al3+ cations are in tetrahedral and half are in octahedral coordination. As one considers andalusite and then kyanite, it is the coordination of the Al3+ cations that characterizes the structural differences. In andalusite, which is also orthorhombic in structure, the AlO6 octahedra chains are linked by SiO4 tetrahedra and AlO5 polyhedra alternate within the structure. Proceeding to kyanite, the chains are linked by SiO4 tetrahedra and AlO6 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 AlO6 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 structures. 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 AlO6 octahedra. Such distinctive structural differences 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 AlO6 octahedral chain structure is preserved during the decomposition. The mullite structure is, however, somewhat complicated by its extensive stability over a wide range of stoichiometries. The composition of mullite can be expressed as
Al(Al1+^Si№ ) d)
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 AlO6 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.