Sillimanite is a polymorph of andalusite and kyanite.  These three minerals share the same formula, but are stable at different P/T conditions.

Physical Properties
Chemical formula AlSi2O5
Class Nesosilicate
Isolated tetrahedra
Crystal system Orthorhombic
Habit Acicular
Long, slender prisms
Subparallel aggregates and splays
Fine-grained fibrous mats
Color White to brown
Hardness 6 to 7
Specific gravity 3.23
Cleavage Perfect (010) (rare)
Fracture Uneven
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Colorless
High relief
XPL High 2nd order colors
δ 0.022
after Perkins, 345-346

Sillimanite in Hand Sample

Sillimanite (brown) in kaolinite

Sillimanite in Thin Section

Thin Section GigaPans

Sillimanite in sillimanite-garnet gneiss, plane polars
Sillimanite in sillimanite-garnet gneiss, crossed polars

Sillimanite, PPL

Sillimanite, XPL

Further Reading

Sillimanite at
Sillimanite at