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Cordierite

A species of Minerals, Also known as Esmarkite (of Berzelius), Jolite, Luchsapphir, Pacific Cat's Eye, Spanish Lazulite, Tanolite

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.

Hardness:

7 - 7.5

Density:

2.505 g/cm³

General Info About Cordierite

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Physical Properties of Cordierite

Colors

Grey, blue, blue-violet, greenish, yellowish brown, colourless to very light blue

Streak

White

Hardness

7 - 7.5 , Extremely hard

Density

2.505 g/cm³, Normal Weight

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Chemical Properties of Cordierite

Formula

(Mg,Fe)₂Al₃(AlSi₅O₁₈)

Elements listed

Al, Fe, Mg, O, Si

Common Impurities

Mn,Fe,Ti,Ca,Na,K

Optical Properties of Cordierite

Refractive Index

1.542-1.578

Birefringence

0.008-0.012

Pleochroism

Strong trichroism: colorless to yellowish, pale blue, dark (violet)-blue

Dispersion

0.017

Optical Character

Biaxial positive or negative

Characteristics of Cordierite

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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.

Cultural Significance of Cordierite

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The Meaning of Cordierite

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.