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Cordierite  (Iolite)
Current inventory:  0 gems
 

Chemistry:  (Mg,Fe)₂ Al₄Si₅O₁₈  [Magnesium Aluminum Silicate]

Discovered in 1813 ;   IMA status: Valid (pre-IMA; Grandfathered).
The name Cordierite is after Pierre Louis A. Cordier (1777-1861), a French mining engineer and geologist who first studied this species. The name Iolite is from the Greek ios for violet and lithos for stone. Cordierite is the IMA recognized name for this mineral species.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/E.12-40

8 : Silicates
E : Cyclosilicates, triplet rings [Si3O9]6-
12 : Beryl series

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

Crystals short prismatic, striated, to 18 cm; typically granular to compact, massive.

Twinning:

Common, simple, lamellar, cyclical.

Physical Properties

Cleavage:

[100] Imperfect/Fair, [001] Poor, [010] Poor

Fracture:

Conchoidal, Sub-Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

7.0 - 7.5

Density:

2.60 - 2.66 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, Pale blue, Violet, Yellow, Gray

Transparency:

Transparent to Translucent

Luster:

Vitreous (Glassy)

Refractive Index:

1.527 - 1.578 Biaxial ( - )

Birefringence:

0.0110 - 0.0180

Dispersion:

0.017; Weak; r < v

Pleochroism:

Strong; X = pale yellow, green; Y = violet, blue-violet; Z = pale blue

Occurances

Geological Setting:

In thermally metamorphosed argillaceous sediments and high-grade regionally metamorphosed schists, gneisses, and granulites; in mafic igneous rocks and granites; detrital.

Type Locality:

Großer Arber Mt., Bayerisch Eisenstein, Zwiesel, Bavarian Forest, Lower Bavaria, Bavaria, Germany

Year Discovered:

1813

View mineral photos:

Cordierite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Cordierite is also called Iolite, Dichroite or even "water sapphire" due to its frequent sapphire blue colors. Although Iolite is the widely used name for this gem, Cordierite is the IMA recognized name for the mineral species.  Iolite has intense and distinct pleochroism showing colors of blue, violet, smokey blue and gray when viewed from different angles. Iolite's crystal structure is very similar to that of Beryl and is a member of the Beryl series of minerals. Iolite rarely contains inclusions of hematite plates that create a beautiful schiller effect.

Although Cordierite is found in many locations worldwide, relatively few produce fine, gem-quality crystals. Some of the better known sources of Iolite gems are Coroaci, Minas Gerais, Southeast Region, Brazil; around Tiruchchirappalli and Coimbatore, Tamil Nadu, India; Tranomaro area, Amboasary Department, Tuléar Province, Madagascar; and from the gem gravels of Sri Lanka.
 

  
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