metamorphic
Iolite-bearing Gneiss (Cordierite)
Cordierite (Mg2Al4Si5O18) in Gneiss matrix
Hardness: 7-7.5 (cordierite); Color: Blue to violet-blue crystals embedded in a tan or grey matrix; Luster: Vitreous to greasy; Crystal structure: Orthorhombic; Specific Gravity: 2.58–2.66.
- Hardness
- 7-7
- Luster
- Vitreous to greasy
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Physical properties
Hardness: 7-7.5 (cordierite); Color: Blue to violet-blue crystals embedded in a tan or grey matrix; Luster: Vitreous to greasy; Crystal structure: Orthorhombic; Specific Gravity: 2.58–2.66.
Formation & geological history
Formed via regional metamorphism of pelitic (clay-rich) rocks under medium to high-grade conditions. Often found in contact metamorphic zones and gneissic terrains of Proterozoic age.
Uses & applications
Used primarily as a gemstone (iolite) when transparent crystals are found. In specimen form, it is highly valued by mineral collectors. Coarser cordierite has industrial uses in ceramics and catalytic converter substrates due to thermal shock resistance.
Geological facts
Iolite is famously known as the 'Viking's Compass.' Because of its extreme pleochroism—appearing blue from one angle and clear/yellow from another—Viking navigators used thin slices to locate the sun on overcast days by polarizing the light.
Field identification & locations
Identify in the field by looking for the distinct violet-blue rectangular crystals within a foliated rock matrix. Common locations include South India (Madras), Sri Lanka, Madagascar, and the Northwest Territories of Canada.
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