3 The Sillimanite Minerals: Andalusite, Kyanite, and Sillimanite 43their most distinguishing feature is the systematic change of coordination of the Al3+
cations from one structure to another. Only kyanite, which is triclinic, has a unit cell
in which the axes are not orthogonal.2 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 decompo-
sition. 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 AlO 5 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 dif-
ferent 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 AlO 6 octahedra. Those
chains are linked or connected by alternating AlO 4 and SiO 4 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
Table 1Some characteristics of the sillimanite minerals and mullite
Properties Kyanite Andalusite Sillimanite Mullite
Formula AlVIAlVISiIVO 5 AlVIAlVSiIVO 5 AlVIAlIVSiIVO 5 AlVI(Al1+2xSi1–2x)IVO 5
Crystallography Triclinic Orthorhombic Orthorhombic Orthorhombic
Unit cell (nm)
a 0.712a 0.779 0.748 0.754
b 0.785 0.790 0.767 0.769
c 0.557 0.555 0.577 0.289
Crystal habit Lathlike Prismatic Fibrous Acicular
Density (Mg m−3) 3.5~3.7 3.1~3.2 3.2~3.3 3.1~3.3
Mohs 5½~7 6½~7½ 6½~7½ 6~7
Hardness {100} pf {110} gd {010} pf {010} pf
Cleavage {010} gd
Refractive index
a 1.710–1.718 1.633–1.642 1.653–1.661 1.630–1.670
b 1.719–1.724 1.639–1.644 1.657–1.662 1.636–1.675
g 1.724–1.734 1.644–1.650 1.672–1.682 1.640–1.690
Tdecomposition ~1,150–1,350°C ~1,250–1,500°C ~1,400–1,700°C N/A
Tdecomp.interval ~200°C ~250°C ~300°C N/A
∆Vdecomposition ~+15% ~+4% ~+8% N/A
Rejected SiO 2 Cristobalite Amorphous Amorphous N/A
aThe triaxial angles for the triclinic Kyanite are a = 89.98°, b = 101.12°, and g = 106.01°