metamorphic
Cordierite in Matrix (Iolite)
Magnesium iron aluminium cyclosilicate (Mg,Fe)2Al4Si5O18
Hardness: 7-7.5. Color: Blue, violet, grayish-blue. Luster: Vitreous/greasy. Crystal structure: Orthorhombic. Strong pleochroism. Specific gravity: 2.58-2.66.
- Hardness
- 7-7
- Color
- Blue, violet, grayish-blue
- Luster
- Vitreous/greasy
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Physical properties
Hardness: 7-7.5. Color: Blue, violet, grayish-blue. Luster: Vitreous/greasy. Crystal structure: Orthorhombic. Strong pleochroism. Specific gravity: 2.58-2.66.
Formation & geological history
Forms through the contact or regional metamorphism of argillaceous rocks (shales and clays). Often found in hornfels or gneisses. Common in Precambrian metamorphic terranes.
Uses & applications
Used as a gemstone (Iolite), in the manufacture of catalytic converters (cordierite ceramics) due to high thermal shock resistance, and for geological research.
Geological facts
Known as the 'Viking's Compass' because its strong pleochroism allowed ancient navigators to determine the direction of the sun on overcast days.
Field identification & locations
Identify by its characteristic violet-blue color and extreme pleochroism (changing color when viewed from different angles). Commonly found in Sri Lanka, India, and Wyoming, USA.
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