orthorhombic Boroaluminosilicates

Kornerupine

(Mg,Fe2+)4Al6(Si,Al,B)5O21(OH)

Crystal Structure

CDL Notation

 #! Species: Kornerupine
#! System: Orthorhombic (mmm)
#! Habit: Elongated prismatic with strong trichroism
orthorhombic[mmm]:{110}@1.0 + {010}@0.6 + {001}@0.4

Crystal System orthorhombic

Point Group mmm

Crystal Forms (3)

{110} {010} {001}

Quick Facts

Hardness

6.5

Specific Gravity

3.27

Refractive Index

1.661

Optical Character

Biaxial -

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Physical Properties

Crystal Systemorthorhombic

Hardness (Mohs)6.5

Specific Gravity3.27

CleavageImperfect parallel to prism

FractureConchoidal to uneven

LustreVitreous

Optical Properties

Refractive Index1.661

Birefringence0.010-0.013

Optical CharacterBiaxial -

Pleochroismstrong: Green / Yellow (Third direction red-brown; Sri Lanka material may show green/colourless/yellow-brown trichroism)

Colours

GreenYellowish-greenBrownRarely colourless

Localities

Sri Lanka (primary; alluvial gravels, Elahera, Ratnapura)Madagascar (gem-quality transparent green)Tanzania (gem material; cat's-eye variety)Myanmar (Burma; minor)Greenland (type locality; large crystals mostly opaque; named for geological explorer Kornerup)

Common Inclusions

[object Object]Liquid inclusions; growth planesOccasional mineral inclusions

Known Treatments

None known in trade

Crystal Forms

prismpinacoid

Diagnostic Features

Strong trichroism (green/yellow/red-brown) — primary diagnostic; clearly visible in dichroscope; biaxial negative; SG 3.27-3.45; RI 1.661-1.683; cat's-eye variety from Sri Lanka and Tanzania

Notes

Mg-Al-Fe boroaluminosilicate; orthorhombic mmm; complex formula with IMA species kornerupine (<=0.5 B per formula unit) and prismatine (>0.5 B; separate IMA species since 1997). Optical constants and cat's-eye kornerupine from Sri Lanka confirmed by Zwaan 1996, Gems & Gemology 32(4):262-269 (DOI 10.5741/gems.32.4.262) [VERIFIED]. Elahera gem field confirmed by Gunawardene & Rupasinghe 1986 (DOI 10.5741/gems.22.2.80) [VERIFIED]. Gem localities by Shigley et al. 2000 (DOI 10.5741/gems.36.4.292) [VERIFIED]. Strong trichroism (green/yellow/red-brown) is the primary field diagnostic; "immediately recognisable under the dichroscope." Colour cause: Fe2+ (green); Cr3+ in intensely green stones. Cat's-eye variety: fibrous hollow tubes parallel to c-axis.