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Huge Blocky Cordierite Crystals
SILICATES; CYCLOSILICATES · ORTHORHOMBIC

Cordierite

Mg2Al4Si5O18

Hardness
7–7.5
Streak
Colorless
Spec. Gravity
2.6–2.7
System
Orthorhombic

About

Cordierite is a strongly pleochroic mineral, and its color will be noticeably different when viewed at different angles. It is one of the few minerals that exhibits such strong pleochroism, and is the most well-known mineral displaying this optical property. In its most typical habit, when a transparent Cordierite specimen is viewed through one angle, it will be violet-blue to blue, and when shifted it will turn gray or yellowish.
Cordierite forms a solid solution series with the rare mineral Sekaninaite. Cordierite is the magnesium-rich end member, and Sekaninaite is the iron-rich end member. Pure Cordierite without any iron present is not common.
Cordierite often is replaced by other minerals, especially phyllosilicates such as micas, Chlorite, and Talc. An interesting and unique habit is the Muscovite pseudomorphs after Cordierite from Japan, which form glittering, flower-shaped trapiche crystals.
Cordierite is named after Louis Cordier (1777-1861), a French geologist and mineralogist who was a founder of the French Geological Society.
For additional information, see the gemstone section on Iolite.

Crystal Forms & Aggregates

Striking Features

Environment

Varieties

✓ common  · ✗ uncommon

Polymorphs

Uses

The transparent gem variety of Cordierite is known as Iolite, and has recently become a mainstream gemstone. Due to its good color and hardness, it is used as an affordable replacement to Sapphire, and is used in ring, earrings, and as pendants.

Noteworthy Localities

Very large, altered Cordierite crystals once came from the Silberberg Mine, Bodenmais, Bavaria, Germany. Brownish-red to tan microcrystals are found in Eifel Mountains of Germany at the Bellerberg volcano in Ettringen. Large, well-formed crystals of Cordierite with very good color were recently found in Søndeled, Risør, Aust-Agder, Norway. Also in Norway is Tvedestrand, Aust-Agder, where Cordierite occurs in a Quartz matrix.
Trapiche-shaped flowers formed from Muscovite pseudomorphs after Cordierite are well known from Kameoka City, Kyoto Prefecture, Japan. Gem-quality Cordierite is found in several of the Madagascar pegmatites, especially in the Tranomaro area, Tuléar Province. Gem crystals also occur in Babati, Manyara Region, Tanzania; and across the Atlantic at Virgolândia, Doce Valley, Minas Gerais, Brazil.
In the U.S., the most prominent localities are in New England. Large, opaque crystals of excellent form are found in a Talc quarry in Richmond, Cheshire Co., New Hampshire. Bluish-gray Cordierite has come from Haddam, Middlesex Co., Connecticut; and a deep blue Cordierite from Hungry Hill, Guilford, New Haven Co., Connecticut.

Common Mineral Associations

Distinguishing Similar Minerals

Photos

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Crystal forms

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Prismatic
Prismatic with Modified Termination