- a strongly dichroic blue mineral consisting of a silicate of magnesium, aluminum, and iron: common in metamorphic rocks.
- a grey or violet-blue dichroic mineral that consists of magnesium aluminium iron silicate in orthorhombic crystalline form and is found in metamorphic rocks. Formula: (Mg,Fe) 2 AL 4 Si 5 O 18 .nH 2 O
