Labradorite – Plagioclase Feldspar

Labradorite mineral species in the plagioclase series of the feldspar group; triclinic crystal system; chemical composition of plagioclase feldspars varies between NaAlSi3O8 and CaAl2SiO8.

Appearance:

• Transparent to opaque
• Labradorite – typically gray to nearly black bodycolor; also colorless, green, yellow, or
ange to brown or brownish red
• Phenomena: labradorescence,aventurescence; sometimes weak chatoyancy or asterism in labradorite

Identification:

• Optic Character: DR, biaxial; labradorite – positive, oligoclase – negative: ACC.
• Tion common

Refractive Index:

• Labradorite – 1.559-1.568 (+.005)
• Oligoclase – 1.537-1.547 (+.004 -.006)
• Birefringence: .007 to .010, labradorite usually .009
• Pleochroism: usually none; yellow – colorless and light yellow
• Ultraviolet Fluorescence: usually inert, may be weak patchy white (LW and SW). Absorption Spectra: not diagnostic
• 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)
• Oligoclase – 2.65 (+.02, -.03)
• Polish Luster: vitreous
• Fracture: uneven to splintery
• Luster: vitreous to pearly
• Cleavage: perfect and easy in two directions; parting is also common

Identifying Characteristics:

• Labradorite – repeated twinning; black, needle-like inclusions
• Sunstone – reddish to golden metallic-looking platelets

Enhancements:

None

Transparent labradorite from:

 • beryl – RI (possibly), optic character/sign, magnification, cleavage
 • quartz – RI, optic character, cleavage, magnification, SG (possibly)
 • scapolite – birefringence, optic character/sign, RI (possibly), fluorescence (posswy
sunstone from
• aventurine quartz – fracture, cleavage, appearance, spectrum, RI (possibly)
• dyed quartz – fracture, cleavage, appearance, magnification, spectrum, RI (possi • goldstone glass – fracture, cleavage, appearance, magnification, optic character • chalcedony – fracture, cleavage, phenomenon, RI (possibly)
• Ptic character (possibly)

Cleaning Methods:

• Ultrasonic: never
• Steamer: never
• Warm soapy water: safe.
• Hardness: 6-61/2
• Toughness: poor

Stability:

• Reaction to Heat: may crack or cleave
• Stability to Light: stable
• Reaction to Chemicals: rapidly attacked by hydrofluoric acid, slowly attacked by
hydrochloric acid

Market:

• Availability: labradorite – plentiful (with labradorescence) to limited (sunstone and
transparent); oligoclase – scarce
• Public Recognition: unfamiliar

Major Sources:

• Finland, Canada, US. Others: Kenya, Madagascar

Recommended Disclosures:

• Avoid rough handling and heat. Advise that it requires care in setting.