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The Mineral cordierite

Huge Blocky Cordierite Crystals

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.

Chemical Formula

Mg₂Al₄Si₅O₁₈

Color

Light to dark blue, purplish-blue, brown, yellow, gray. Rarely red to brownish-red. Highly pleochroic, showing a different color when rotated on an angle.

Crystal System

Orthorhombic

Properties

Streak
Colorless

Hardness
7 - 7.5

Transparency
Transparent to opaque

Specific Gravity
2.6 - 2.7

Luster
Vitreous

Cleavage
3,1

Fracture
Conchoidal to uneven

Tenacity
Brittle

Crystal Habits

Crystals, which are uncommon, are wide and stubby or prismatic. Crystals will usually form in a pseudohexagonal shape, and frequently have etches or striations. Cordierite is most often grainy, massive, and in unshaped fragments. It is also found as rounded waterworn pebbles.

3D Crystal Atlas

Additional Information

Composition
Magnesium aluminum silicate, almost always with some iron.

In Group
Silicates; Cyclosilicates

Striking Features
Obvious pleochroism in transparent crystals and relatively high hardness

Environment
In contact metamorphic rock such as hornfels and gneiss, in altered Serpentine environments, and in course granites.

Rock Type
Igneous, Metamorphic

Other Names

	Dichroite
	Water Sapphire

Varieties

Cerasite

- A Muscovite pseudomorphs after Cordierite/Indialite from Japan, which forms in highly attractive, flower-shaped trapiche crystals. These are known in Japanese as sakura ishi - meaning "cherry blossom stones".

Gigantolite

- Mica that forms a pseudomorph after large Cordierite crystals.

Iolite

- Gem form of Cordierite. May also be used as a synonym for Cordierite.

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.