- 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 OAlso called: dichroite, iolite
- A light-blue to dark-blue or gray orthorhombic mineral. Cordierite is a silicate of magnesium, aluminum, and sometimes iron, and is found in granites and in metamorphic rocks that form under relatively low-pressure conditions. Chemical formula: (Mg,Fe)2Al4Si5O18.