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Labradorite is a tectosilicate mineral, a calcium-sodium feldspar, belonging to the plagioclase series. Its most distinctive feature is a schiller effect, known as labradorescence, which displays a brilliant iridescent play of colors (typically blue, green, yellow, and red) when light strikes the mineral at certain angles. This optical phenomenon is caused by light interference within microscopic lamellar exsolution intergrowths of albite and orthoclase within the crystal structure. The composition of labradorite falls within the range of An50 to An70 (50-70% anorthite, 30-50% albite).
How to Identify
- Color
- Typically gray, dark gray, greenish-gray, or brownish-gray in ordinary light. Displays iridescent flashes of blue, green, yellow, orange, red, and purple (labradorescence) when rotated.
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Often massive, granular, or in tabular crystals. Can exhibit polysynthetic twinning striations on cleavage surfaces.
- Crystal Form
- Triclinic system, typically forming tabular to blocky crystals. Often found as anhedral to subhedral grains in igneous rocks.
- Cleavage
- Perfect in two directions, nearly at right angles (approximately 86 degrees and 94 degrees), characteristic of plagioclase feldspars.
- Geological Environment
- Commonly found in mafic igneous rocks such as anorthosite, gabbro, basalt, and norite. Also occurs in some high-grade metamorphic rocks.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.68 to 2.72
- Crystal System: Triclinic
- Color: Gray, dark gray, greenish-gray, brownish-gray; exhibits labradorescence (iridescent play of colors)
- Luster: Vitreous to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°)
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.5-0.3)Al(Si1.5-1.7Al0.5-0.3)O8
Quick Check
- Color: Gray to dark gray with iridescent flashes of blue, green, yellow, etc.
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to blocky crystals. Often twinned (polysynthetic twinning).
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.
Formation
Labradorite forms primarily in mafic igneous rocks such as basalt, gabbro, and anorthosite. It crystallizes from magma rich in calcium and sodium, typically under conditions of slow cooling, allowing for the development of larger crystals. It can also be found in some metamorphic rocks, particularly those derived from mafic protoliths.
Usage
Primarily used as a gemstone and ornamental stone due to its striking labradorescence. It is carved into cabochons, beads, and sculptures. Less commonly, it is used as a facing stone in architecture. It has no significant industrial uses beyond its aesthetic appeal.
Age Distribution
Found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic.
Where to Find
Labrador, Canada
The original and type locality, where it was first discovered in 1770 by Moravian missionaries. Large deposits of anorthosite containing labradorite are found here.
Finland
Known for producing a high-quality variety called Spectrolite, which exhibits a full spectrum of colors. Found in anorthosite intrusions.
Madagascar
Significant source of gem-quality labradorite, often with strong labradorescence.
Norway
Occurrences in various igneous and metamorphic settings.
Russia
Deposits found in the Ural Mountains and other regions.
United States
Found in various states, including Oregon (often with aventurescence, sometimes called Oregon Sunstone), New York (Adirondack Mountains anorthosite), and others.
Australia
Occurrences in igneous rocks.
Finding Tips
Look in Mafic Igneous Rocks
Focus your search on areas with known occurrences of anorthosite, gabbro, and basalt. Labradorite is a primary constituent of these rock types.
Examine Cleavage Surfaces
The characteristic labradorescence is best observed on freshly broken or polished surfaces. Look for the iridescent play of colors when rotating the sample under a light source.
Check for Polysynthetic Twinning
Fine parallel striations (twinning lamellae) on cleavage surfaces are a strong indicator of plagioclase feldspar, including labradorite.
Consider Associated Minerals
Labradorite often co-occurs with pyroxenes (e.g., augite), olivine, and magnetite in mafic igneous rocks.
Safety Precautions
Labradorite itself is not hazardous. However, when collecting in the field, always wear appropriate safety gear (gloves, eye protection) and be aware of your surroundings, especially in quarry or outcrop environments. Avoid inhaling dust if cutting or grinding samples, as fine silica dust can be a respiratory irritant.
Similar Rocks
Moonstone
Orthoclase or Albite (with lamellar intergrowths)
Also known as: Adularia, Selenite (historical, now refers to gypsum)
Sunstone
Oligoclase or Labradorite (with hematite or goethite inclusions)
Also known as: Heliolite
Andesine
Andesine (a variety of plagioclase feldspar)
Also known as: Andesine-Labradorite
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with a composition range of An50 to An70 (50-70% anorthite, 30-50% albite).
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