Cordierite
Mineral (Mg,Fe)2Al4Si5O18
Also called Dichroite and Water sapphire
About it
Violet-blue grains that look grey or yellowish from another side (strong pleochroism); gem iolite comes from India, Sri Lanka, and Madagascar, and grains are common in gneiss.
- Type
- Mineral
- Group
- Silicate (cyclosilicate), cordierite group
- Formula
- (Mg,Fe)2Al4Si5O18
- Hardness (Mohs)
- 7–7.5
- Specific gravity
- 2.53–2.78
- Streak
- white
- Lustre
- vitreous (glassy), greasy
- Crystal system
- Orthorhombic
- Colours
- blue, violet, grey, yellowish
- Transparency
- transparent, translucent
- Cleavage and fracture
- poor; conchoidal fracture
Typical values: real specimens vary. For more, see mindat.org.