Labradorite – Plagioclase Feldspar

Appearance:

• Labradorite mineral species in the plagioclase series of the feldspar group; triclinic crystal system; chemical composition of plagioclase feldspars varies between NaAlSi3O8 and CaAl2SiO8
• Transparent to opaque
• Labradorite – typically gray to nearly black body color; also colorless, green, yellow, or
ange to brown or brownish red
• Phenomena: labradorescence, aventurescence; sometimes weak chatoyancy or asterism in labradorite

Variety and Trade Names:

• Spectrolite – Trade name for labradorite with strong color flashes (labradorescence)

Identification:

• Optic Character: DR, biaxial; labradorite – positive, oligoclase – negative: ACC. reation common
• Refractive Index: 1.559 -1.568 (+.005)
• Birefringence: .007 to .010, labradorite usually .009
• Pleochroism: usually none; yellow – colorless and light yellow
• Cause of Color: sunstone – sheen colors from metallic-looking inclusions such as native copper or hematite; labradorite – labradorescent colors (typically blues and greens) due to interference of light reflecting off finely layered structure
• Specific Gravity: labradorite – 2.70 (1.05)
• Polish Luster: vitreous
• Fracture: uneven to splintery
• Luster: vitreous to pearly
• Cleavage: perfect and easy in two directions; parting is also common
• Hardness: 6-61/2
• Toughness: poor

Sources:

• Finland, Canada, Australia, Russia, Kenya, Madagascar.