Cordierite is a cyclosilicate of magnesium and aluminium. Iron is almost always present and a series exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)₂Al₄Si₅O₁₈ to (Fe,Mg)₂Al₄Si₅O₁₈.

Cordierite typically occurs in contact or regional metamorphism of argillaceous rocks. Common associated minerals include: sillimanite, spinel, plagioclase, orthopyroxene, garnet, and anthophyllite. Cordierite also occurs in some granite and pegmatites. Alteration products include mica, chlorite, or talc.

Cordierite is named after a French geologist P. L. A. Cordier (1777 - 1861).

As the transpaent variety iolite, it is often used as a gemstone. The name iolite comes from the Greek word for violet. Another old name is dichroite a greek word meaning two colored rock a reference to Cordierite's strong pleochroism. Gem quality iolite is sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. Gem quality cordierite is found in various places such as Sri Lanka and Burma. Other localities where iolite is found include Namibia, Brazil, Tanzania, Madagascar, Connecticut, and the Yellowknife area of the Northwest Territories of Canada.

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p. 395-396, ISBN 0471805807
• Webmineral
• Mindat.org
• Mineral galleries