Three minerals have a chemical composition of Al2SiO5. These are kyanite, andalusite, and sillimanite. Kyanite is the high-pressure polymorph, sillimanite forms at high temperature, and andalusite is the low-pressure polymorph.
| Physical Properties of Kyanite | |
| Chemical Classification | Silicate |
| Color | Blue, white, gray, green, colorless |
| Streak | White, colorless |
| Luster | Vitreous, pearly |
| Diaphaneity | Transparent to translucent |
| Cleavage | Perfect in two directions, faces sometimes striated |
| Mohs Hardness | Kyanite often occurs in long, bladed crystals. These have a hardness of 4.5 to 5 along the length of the crystals and 6.5 to 7 across the width of the crystals. |
| Specific Gravity | 3.5 to 3.7 |
| Diagnostic Properties | Color, cleavage, bladed crystals |
| Chemical Composition | Al2SiO5 |
| Crystal System | Triclinic |
| Uses | Ceramics, gemstones |