orthorhombic Nesosilicates

Andalusite (Chiastolite (Andalusite variety))

Al2SiO5

Crystal Structure

CDL Notation

 #! Species: Chiastolite (carbonaceous Andalusite)
#! System: Orthorhombic (mmm)
#! Habit: Prismatic with cross-shaped carbonaceous inclusions
orthorhombic[mmm]:{110}@1.0 + {010}@0.7[inclusion:carbonaceous_cross] + {001}@0.9

Crystal System orthorhombic

Point Group mmm

Crystal Forms (3)

{110} {010} {001}

Quick Facts

Hardness

7.0

Specific Gravity

3.13

Refractive Index

1.629

Optical Character

Biaxial -

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Physical Properties

Crystal Systemorthorhombic

Hardness (Mohs)7.0

Specific Gravity3.13

CleavageDistinct prismatic {110} in two directions at ~90°

FractureUneven to conchoidal

LustreVitreous

Optical Properties

Refractive Index1.629

Birefringence0.007-0.013

Optical CharacterBiaxial -

Dispersion0.016

Pleochroismstrong: Dark red to reddish-brown / Olive green to yellowish-green (Third direction orange to brownish-yellow; trichroism visible in dichroscope; "The strong trichroism of andalusite — with dark red, olive green, and orange-yellow visible through the dichroscope — is the single most diagnostic optical feature of this species")

Colours

Yellow-greenOlive greenBrownish-redReddish-orangeGreenish-brown

Localities

Brazil (Minas Gerais; primary source of transparent gem-quality)Sri Lanka (alluvial; lighter tone)Spain (Almeria, Castile; chiastolite variety)MadagascarUSA (California; chiastolite variety)

Common Inclusions

[object Object][object Object]Apatite or mica inclusions (metamorphic paragenesis)

Known Treatments

Generally not treated

Crystal Forms

prism

Diagnostic Features

Strong trichroism (dark red/reddish-brown + olive green to yellowish-green + orange to brownish-yellow) — the single most diagnostic optical feature; chiastolite cross-section (carbonaceous inclusions in symmetrical cross pattern in some crystals)

Notes

Al2SiO5 nesosilicate; one of three Al2SiO5 polymorphs (andalusite, kyanite, sillimanite). Orthorhombic mmm; biaxial negative with 2V ~85°. Strong trichroism (X = dark red, Y = olive green, Z = orange-yellow) is primary diagnostic. Chiastolite is the carbonaceous variety showing a symmetrical cross in cross-section. Gemmological constants confirmed by Fernandes & Choudhary 2009, Gems & Gemology 45(2):120-129 (DOI 10.5741/gems.45.2.120) [VERIFIED]. Colour cause: Fe3+ substitution producing pleochroic absorption pattern. Two cleavage directions at ~90°; avoid hard impacts. Standard gem material from Brazil; chiastolite as collector's item from Spain.