andalusite.yaml

id: andalusite
name: Andalusite
crystal_system: orthorhombic
point_group: mmm
chemistry: Al2SiO5
mineral_group: Al2SiO5 polymorph group (with kyanite and sillimanite)
category: Nesosilicates
origin: natural
hardness: 7-7.5
sg: 3.13-3.17
ri: 1.629-1.650
birefringence: 0.007-0.013
optical_character: Biaxial -
dispersion: 0.016
lustre: Vitreous
cleavage: Distinct prismatic {110} in two directions at ~90°
fracture: Uneven to conchoidal
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)"
pleochroism: Strong trichroism
pleochroism_strength: strong
pleochroism_color1: Dark red to reddish-brown
pleochroism_color2: Olive green to yellowish-green
pleochroism_notes: 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"
colors:
- Yellow-green
- Olive green
- Brownish-red
- Reddish-orange
- Greenish-brown
treatments:
- Generally not treated
localities:
- Brazil (Minas Gerais; primary source of transparent gem-quality)
- Sri Lanka (alluvial; lighter tone)
- Spain (Almeria, Castile; chiastolite variety)
- Madagascar
- USA (California; chiastolite variety)
inclusions:
- Chiastolite: carbonaceous (graphite-rich) inclusions forming the diagnostic cross pattern
- Gem-quality: occasional two-phase fluid inclusions
- Apatite or mica inclusions (metamorphic paragenesis)
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.
expressions:
- slug: default
  name: Andalusite
  cdl: |
    #! Species: Andalusite
    #! System: Orthorhombic (mmm)
    #! Habit: Coarse prismatic with square cross-section
    orthorhombic[mmm]:{110}@1.0 + {010}@0.7 + {001}@0.4
  is_primary: true
  form_description: >
    Coarse prismatic crystals with near-square cross-section; {110} prism dominant. Strong trichroism
    (red/olive-green/orange-yellow) is the diagnostic optical feature. Gem quality from Brazil
    (Minas Gerais). Biaxial negative with 2V ~85°.
  forms:
  - prism
  - pinacoid
- slug: chiastolite
  name: Chiastolite (Andalusite variety)
  cdl: |
    #! 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]
  is_primary: false
  form_description: >
    Variety of andalusite containing carbonaceous (graphite-rich) inclusions arranged in a
    symmetrical cross parallel to the prism faces; visible in cross-section cabochons.
    Grey, brown, and white body colour. Spain and California localities.
  forms:
  - prism