Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Labradorite is a calcium-rich member of the plagioclase feldspar series, characterized by its distinctive iridescent optical effect known as labradorescence. This phenomenon displays a brilliant play of colors, typically blues, greens, yellows, and reds, as light interacts with microscopic exsolution lamellae within the mineral structure. The base color of labradorite is usually dark gray, black, or greenish-gray, which serves as a backdrop for the vibrant flashes of color.
How to Identify
- Color
- Typically dark gray, black, or greenish-gray, with iridescent flashes of blue, green, yellow, and red (labradorescence).
- Luster
- Vitreous to pearly.
- Texture
- Typically massive, granular, or in tabular crystals within host rocks.
- Crystal Form
- Usually anhedral to subhedral grains, but can form tabular, prismatic crystals. Twinning (polysynthetic) is common and often visible as fine parallel lines on cleavage surfaces.
- Cleavage
- Perfect in two directions at nearly 90 degrees (86° and 94°), characteristic of plagioclase feldspars.
- Geological Environment
- Mafic igneous rocks (basalt, gabbro, anorthosite), some metamorphic rocks, and as detrital grains in sediments.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale.
- Specific Gravity: 2.68 to 2.72.
- Crystal System: Triclinic.
- Color: Dark gray, black, or greenish-gray, with characteristic iridescent blue, green, yellow, and red flashes.
- Luster: Vitreous to pearly.
- Transparency: Translucent to opaque.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions, intersecting at approximately 86° and 94°.
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)4O8 with 50-70% anorthite (CaAl2Si2O8) component.
Quick Check
- Color: Dark gray to black with iridescent flashes (labradorescence).
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Polysynthetic twinning is common.
- Cleavage Type: Perfect basal {001} and good prismatic {010} cleavage, intersecting at nearly 90 degrees.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to 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 at high temperatures and pressures. 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 unique labradorescence. It is carved into cabochons, beads, and sculptures. High-quality specimens are highly prized in jewelry. Less commonly, it can be a component of dimension stone (e.g., anorthosite).
Age Distribution
Found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic.
Where to Find
Labrador, Canada
The type locality, where it was first discovered in 1770. Large deposits of anorthosite containing labradorite are found here.
Finland
Known for producing high-quality labradorite with a full spectrum of colors, often called 'Spectrolite'.
Norway
Significant deposits, particularly in the Larvik area, where it is a major component of the igneous rock Larvikite (a monzonite containing abundant labradorite).
Madagascar
Produces good quality labradorite, often with strong labradorescence.
Russia
Deposits found in various regions.
United States
Occurrences in states like Oregon (Sunstone, which is an oligoclase-labradorite), New York (Adirondack Mountains anorthosite), and others.
Australia
Some occurrences reported.
Finding Tips
Look in Mafic Igneous Rocks
Search for labradorite in outcrops of basalt, gabbro, and especially anorthosite. These rocks are typically dark-colored and coarse-grained.
Observe for Labradorescence
The most distinctive feature is the play of colors. Rotate the sample under a light source to observe the iridescent flashes. This effect is often best seen on polished or fractured surfaces.
Check for Cleavage
Examine broken surfaces for two distinct cleavage planes intersecting at nearly 90 degrees. Fine parallel striations (polysynthetic twinning) may also be visible on these surfaces.
Consider Associated Minerals
Labradorite is often found with other mafic minerals like pyroxenes (augite), olivine, and magnetite.
Field Identification
In the field, look for dark, often massive rocks with a distinctive metallic or pearly luster on fresh breaks, and try to catch the iridescent flash by tilting the sample.
Similar Rocks
Andesine
Plagioclase Feldspar (Andesine variety)
Also known as: Red Labradorite (incorrectly applied to some Andesine)
Oligoclase
Plagioclase Feldspar (Oligoclase variety)
Also known as: Sunstone (a variety of Oligoclase)
Moonstone
Orthoclase or Albite-Oligoclase (with adularescence)
Also known as: Adularia, Peristerite
Scientific Classification
- Mineral Class
- Silicate mineral
- Group
- Feldspar group, Plagioclase series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Labradorite specifically contains 50-70% anorthite component (An50-An70).
Explore Labradorite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.