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Huge Blocky Cordierite Crystals

The Mineral cordierite

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
Composition Magnesium aluminum silicate, almost always with some iron.
Variable Formula (Mg,Fe3+)2Al4Si5O18
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.
Streak Colorless
Hardness 7 - 7.5
Crystal System Orthorhombic
3D Crystal Atlas
(Click for animated model) 
Crystal Forms
and Aggregates
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.
Transparency Transparent to opaque
Specific Gravity 2.6 - 2.7
Luster Vitreous
Cleavage 3,1
Fracture Conchoidal to uneven
Tenacity Brittle
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
Popularity (1-4) 2
Prevalence (1-3) 3
Demand (1-3) 2

Cordierite AUCTIONS
Water Sapphire

 -  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".
 -  Mica that forms a pseudomorph after large Cordierite crystals.
 -  Gem form of Cordierite. May also be used as a synonym for Cordierite. 

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.

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.

Quartz, Biotite, Almandine, Andalusite, Corundum, Talc, Anthophyllite

Zoisite - The variety Tanzanite can appear similar, although it has perfect cleavage and is only known from the Arusha area of Tanzania.
Corundum - Greater hardness and lacks strong pleochroism.

cordierite PHOTOS
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