THE MINERAL LABRADORITE


Labradorite is truly a fascinatingly beautiful mineral. Its a mineral whose charm is not fully noticed and may be overlooked if not viewed from the proper position. Generally a dull, dark looking mineral with no special virtue until the colorful shiller is observed glowing on the surface. Labradorite can produce a colorful play of light across cleavage planes and in sliced sections called labradorescence. The usually intense colors range from typical blues and violets through greens, yellows and oranges. Some rare specimens display all these colors simultaneously.

The color display is from lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a separating and layering of these two phases. The resulting color effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers. This refracted ray is slowed by the extra travel through the layers and mixes with other rays to produce a light ray coming out that has a different wavelength than when it went in. The wavelength could correspond to the wavelength of a particular color, such as blue. The effect depends on the thickness and orientation of the layers. If the layers are too thick or too thin no color shiller is seen. Also if the viewer does not observe from the precise angle or if light is not supplied from the proper angle then no color shiller is seen. The labradorescence is truly a one of a kind mineralogical experience and must be observed in person in order to truly appreciate its beauty.

Labradorite is a member of the plagioclase series of minerals. The plagioclase series comprises felspars that range in chemical composition from pure NaAlSi3 O8 to pure CaAl2 Si2 O8 . Labradorite is defined at approximately the 50% to 70% CaAl2 Si2 O8 . Labradorite by definition must contain 50-70% calcium to 50-30% sodium in the sodium/calcium position of the crystal structure. All members of the plagioclase series usually display lamellar twinning called "Albite Twinning". The twinning is caused by a error in the crystal structure during its growth.

PHYSICAL CHARACTERISTICS:

  • Color is gray to smoky black.
  • Luster is dull to vitreous.
  • Transparency crystals are transparent to translucent.
  • Crystal System is triclinic; bar 1
  • Crystal Habits include blocky crystals that rarely form free of the host rock and therefore do not usually show their full crystal forms. In sliced sections of rock, the labradorite appears as blocky chunks with a predominance of near right angled corners. Twinning is common and pervasive so that in labradorite it produces a layered or stacked effect. The twin layers are typically only fractions of millimeters to several millimeters thick. Crystals of labradorite are found imbedded in gabbros and other mafic igneous rocks of low silica content. In anorthosites, labradorite is a main constituent.
  • Cleavage is perfect in one and good in another direction forming nearly right angled prisms.
  • Fracture is conchoidal.
  • Hardness is 6 - 6.5.
  • Specific Gravity is approximately 2.70 - 2.74 (average)
  • Streak is white.
  • Associated Minerals are biotite, pyroxene and hornblende.
  • Other Characteristics: index of refraction is 1.55 - 1.75. Play of colors called labradorescence seen from certain directions can cause flashes of blue, violet and green and sometimes orange and yellow.
  • Notable Occurrences include Labrador, Canada and Scandinavian Peninsula.
  • Best Field Indicators are occurrence, twinning striations and labradorescence.
 

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