Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Iolite, scientifically known as Cordierite, is a cyclosilicate mineral. Its most striking characteristic is its strong pleochroism, meaning it exhibits different colors when viewed from different crystallographic directions. This property gives rise to its common name 'dichroite' (meaning 'two-colored rock') and its gemstone name 'iolite' (from the Greek 'ios' meaning violet). It typically appears in shades of blue, violet, and yellowish-gray. Gem-quality iolite is prized for its sapphire-like blue to violet hues. It is a relatively common metamorphic mineral, often associated with minerals like garnet, sillimanite, kyanite, and biotite.
How to Identify
- Color
- Typically blue, violet-blue, yellowish-gray, brownish-yellow, or colorless. Strong pleochroism is a key identifier, showing different colors (e.g., deep violet-blue, pale blue, yellowish-gray) when rotated.
- Luster
- Vitreous (glassy) to greasy.
- Texture
- Often occurs as disseminated grains, anhedral to subhedral crystals, or columnar aggregates within metamorphic rocks. Gem-quality material is typically transparent to translucent.
- Crystal Form
- Orthorhombic system. Crystals are typically prismatic, often with pseudohexagonal cross-sections due to twinning. Twinning is common, often cyclic or lamellar.
- Cleavage
- Distinct to good cleavage on {010}, poor on {100} and {001}. Fracture is conchoidal to uneven.
- Geological Environment
- Metamorphic rocks (schists, gneisses, hornfels) derived from aluminous sediments. Also found in some granites and pegmatites.
Key Facts
- Hardness: 7-7.5 (Mohs scale)
- Specific Gravity: 2.53-2.78 g/cm³
- Crystal System: Orthorhombic
- Color: Blue, violet-blue, yellowish-gray, brownish-yellow, colorless (strongly pleochroic)
- Luster: Vitreous to greasy
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Distinct to good on {010}, poor on {100} and {001}
- Composition: Magnesium iron aluminum cyclosilicate
Quick Check
- Color: Blue to violet-blue, yellowish-gray (pleochroic)
- Luster: Vitreous to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often pseudohexagonal due to twinning; also massive, granular, or columnar aggregates.
- Cleavage Type: Distinct to good in one direction ({010}), poor in two others ({100}, {001}).
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Cordierite forms primarily during regional metamorphism of argillaceous (clay-rich) rocks, often in the amphibolite and granulite facies. It can also form in contact metamorphic aureoles around igneous intrusions, and less commonly in some igneous rocks (e.g., granites, norites) as a primary crystallization product or through assimilation of aluminous country rock. It is stable at high temperatures and moderate to low pressures.
Usage
Gemstone (faceted stones, cabochons), scientific research (geothermobarometry), and occasionally as a refractory material due to its thermal stability.
Age Distribution
Precambrian to Cenozoic, typically found in metamorphic rocks of various ages.
Where to Find
Sri Lanka
Known for producing gem-quality iolite, often found in alluvial deposits.
India (Odisha)
Significant source of gem-quality iolite, particularly from metamorphic terrains.
Madagascar
Produces fine quality iolite, often associated with other metamorphic minerals.
Brazil
Various occurrences, including gem-quality material.
Norway
Historical source, particularly for larger crystals.
Finland
Known for cordierite occurrences in metamorphic rocks.
Canada (Yellowknife, Northwest Territories)
Notable for large cordierite crystals in metamorphic rocks.
USA (Connecticut, Wyoming)
Occurrences in metamorphic rocks, though gem quality is less common.
Finding Tips
Look in Metamorphic Terrains
Focus your search on areas known for regional or contact metamorphism of clay-rich sedimentary rocks. Schists, gneisses, and hornfels are prime targets.
Identify Associated Minerals
Iolite often co-occurs with minerals like garnet, sillimanite, kyanite, and biotite. The presence of these minerals can indicate a favorable environment for cordierite.
Observe Pleochroism
When examining a potential iolite specimen, rotate it under a light source. The distinct change in color (e.g., from blue to yellowish-gray) is a strong indicator of iolite.
Check Hardness
Perform a scratch test if appropriate and non-damaging to the specimen. Iolite's hardness of 7-7.5 on the Mohs scale can help differentiate it from softer minerals.
Examine Crystal Habit
Look for prismatic crystals, sometimes with pseudohexagonal outlines, or disseminated anhedral grains within the host rock.
Similar Rocks
Sapphire
Corundum (Al2O3)
Also known as: Corundum
Tanzanite
Zoisite (Ca2Al3(SiO4)3(OH))
Also known as: Blue Zoisite
Kyanite
Kyanite (Al2SiO5)
Also known as: Disthene
Scientific Classification
- Mineral Class
- Silicates
- Group
- Cyclosilicates
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2Al4Si5O18
- Composition
- Magnesium iron aluminum cyclosilicate. The ratio of Mg to Fe is variable, leading to a solid solution series. Minor substitutions of Mn, Ca, Na, K, and H2O can also occur.
Explore Iolite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.