metamorphic
Iolite (Cordierite)
Cordierite (Magnesium iron aluminum cyclosilicate), Mg2Al4Si5O18
Hardness: 7-7.5 on Mohs scale. Color: Violet-blue to grayish-blue (pleochroic). Luster: Vitreous/Glassy. Crystal structure: Orthorhombic. Cleavage: Fair (one direction). Specific Gravity: 2.58–2.66.
- Hardness
- 7-7
- Color
- Violet-blue to grayish-blue (pleochroic)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 7-7.5 on Mohs scale. Color: Violet-blue to grayish-blue (pleochroic). Luster: Vitreous/Glassy. Crystal structure: Orthorhombic. Cleavage: Fair (one direction). Specific Gravity: 2.58–2.66.
Formation & geological history
Formed during regional metamorphism of argillaceous (clay-rich) rocks, often found in schists and gneisses, or as an accessory mineral in granitic rocks and pegmatites.
Uses & applications
Used primarily as a gemstone in jewelry (the 'water sapphire') and for industrial ceramics that require high thermal shock resistance.
Geological facts
Iolite is famous for its extreme pleochroism; it can look deep blue from one angle, honey-yellow from another, and clear from a third. Vikings allegedly used thin slices as a polarizing filter to locate the sun on cloudy days.
Field identification & locations
Identify in the field by its distinct violet-blue color and the way its color changes when rotated. Commonly found in India, Sri Lanka, Brazil, and Tanzania. Look for it in high-grade metamorphic terrains.
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