Cordierite
A variety of Minerals
What is Cordierite?
Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings.
Optical Properties
- Refractive Index
- 1.542-1.578
- Birefringence
- 0.008-0.012
- Pleochroism
- Strong trichroism: colorless to yellowish, pale blue, dark (violet)-blue
- Optical Character
- Biaxial positive or negative
- Dispersion
- 0.017
Market Value Factors
Pricing varies for every rock and mineral, so use these universal factors to gauge Cordierite before comparing listings or appraisals.
Size & Weight
Larger, intact specimens usually command higher prices.
Rarity & Demand
Scarce material or popular varieties sell at a premium.
Condition & Finish
Chips, repairs, and heavy wear lower value; clean prep helps.
Treatment & Provenance
Untreated specimens with documented locality are prized.
Cordierite Localities Map
See where Cordierite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.
The Meaning
Cordierite was used by the Vikings to find the direction of the sun on overcast or rainy days and is mentioned in "The Saga of King Olaf". Symbolically, cordierite is the gemstone that is associated with a 21st wedding anniversary.
Key Characteristics
Formation of Cordierite
Cordierite typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs, for example, in the granite contact zone at Geevor Tin Mine in Cornwall.
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Quick Facts
Physical Properties
- Color
- Grey, blue, blue-violet, greenish, yellowish brown, colourless to very light blue
- Hardness (Mohs)
- 7 - 7.5
- Density
- 2.505 g/cm³
- Streak
- White
Chemical Properties
- Chemical Formula
- (Mg,Fe)2Al3(AlSi5O18)
- Elements
- Al, Fe, Mg, O, Si
Also Known As
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Cordierite FAQs
How do I identify Cordierite?
Cordierite can be identified by its hardness of 7 - 7.5 on the Mohs scale, Grey color. Look for these key characteristics when examining specimens.
What color is Cordierite?
Cordierite typically appears in Grey, blue, blue-violet, greenish, yellowish brown, colourless to very light blue. Color can vary depending on impurities and formation conditions.
How hard is Cordierite?
Cordierite has a hardness of 7 - 7.5 on the Mohs scale. This makes it a relatively hard mineral suitable for jewelry.
What is the formation of cordierite of Cordierite?
Cordierite typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs, for example, in the granite contact zone at Geevor Tin Mine in Cornwall.
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