A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Sillimanite is the rarest of the three trimorphs . It is a common fiberous massive form.
Color is white, brown and green.
Luster is silky when fiberous or vitreous otherwise. Transparency crystals are transparent to translucent. Cleavage is good in one direction lengthwise. Fracture is splintery. Hardness is 7.5 in large crystals but more fiberous forms are softer.