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Moonstone is a feldspar mineral known for its characteristic adularescence, a shimmering optical phenomenon resembling moonlight. This effect is caused by the scattering of light by microscopic lamellar intergrowths of two different feldspar species, typically orthoclase and albite, or albite and oligoclase. The most prized moonstones exhibit a blue sheen, though white, gray, peach, and rainbow varieties also exist. It is a relatively soft gemstone, requiring careful handling.
How to Identify
- Color
- Typically colorless to white, but can also be gray, peach, yellow, or blue. The most distinctive feature is the adularescence, a milky or bluish sheen that appears to float just beneath the surface.
- Luster
- Vitreous to pearly.
- Texture
- Smooth when polished (cabochon cut). In its rough form, it can be massive or show distinct crystal faces.
- Crystal Form
- Monoclinic (orthoclase variety) or Triclinic (plagioclase variety). Crystals are typically prismatic, tabular, or blocky, but often found as massive aggregates. Adularia often forms pseudo-orthorhombic crystals.
- Cleavage
- Perfect in two directions, intersecting at or near 90 degrees (for orthoclase) or slightly less than 90 degrees (for plagioclase).
- Geological Environment
- Hydrothermal veins, pegmatites, granites, syenites, and metamorphic rocks. The adularia variety is common in alpine-type fissures and low-temperature hydrothermal deposits. Plagioclase moonstone is found in more diverse igneous and metamorphic settings.
Key Facts
- Hardness: 6-6.5 on the Mohs scale.
- Specific Gravity: 2.56-2.59 (Orthoclase Moonstone); 2.56-2.62 (Plagioclase Moonstone).
- Crystal System: Monoclinic (Orthoclase/Adularia); Triclinic (Plagioclase/Peristerite).
- Color: Colorless, white, gray, peach, yellow, blue; characterized by adularescence.
- Luster: Vitreous to pearly.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions, intersecting at approximately 90 degrees (orthoclase) or slightly less (plagioclase).
- Composition: Potassium aluminum silicate (KAlSi3O8) for orthoclase moonstone; Sodium calcium aluminum silicate ((Na,Ca)Al(Si,Al)Si2O8) for plagioclase moonstone.
Quick Check
- Color: Colorless to white, gray, peach, yellow, blue, with adularescence.
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, tabular, blocky, or massive. Adularia often forms pseudo-orthorhombic crystals.
- Cleavage Type: Perfect in two directions, {001} and {010}.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly.
Formation
Moonstone typically forms in igneous and metamorphic rocks. The adularia variety of orthoclase moonstone forms in low-temperature hydrothermal veins and alpine-type fissures. Plagioclase moonstone (peristerite) forms in pegmatites and other igneous intrusions, as well as in metamorphic rocks. The characteristic adularescence (schiller) is caused by the diffraction of light from alternating layers of two different feldspar minerals (orthoclase and albite, or albite and oligoclase) with slightly different refractive indices. These lamellar intergrowths, often on a sub-microscopic scale, are a result of exsolution during cooling.
Usage
Primarily used as a gemstone in jewelry due to its unique adularescence. It is cut into cabochons to best display this optical effect. Historically, it has been used in various cultures for its perceived mystical properties.
Age Distribution
Moonstone forms in various geological settings and thus does not have a specific age distribution; its formation is tied to the age of the host rocks, which can range from Precambrian to Cenozoic.
Where to Find
Sri Lanka
Historically the most significant source, particularly for fine blue moonstone. Found in pegmatites and alluvial deposits.
India
Known for producing 'rainbow moonstone' (a variety of labradorite with a strong play-of-color, often misidentified as true moonstone) and some white and peach moonstone.
Myanmar (Burma)
Produces high-quality moonstone, often with a strong blue adularescence.
Madagascar
A source of various feldspar minerals, including some moonstone varieties.
Tanzania
Known for some moonstone occurrences.
United States (e.g., New Mexico, Virginia, Pennsylvania)
Minor occurrences of moonstone have been reported in various states, often in pegmatites or hydrothermal veins.
Brazil
Some deposits of moonstone have been found.
Finding Tips
Look for Feldspar-Rich Environments
Moonstone is a feldspar, so search in areas known for pegmatites, granites, syenites, or hydrothermal veins. These are common environments for feldspar crystallization.
Identify Adularescence
The key identifying feature is the adularescence. Rotate potential specimens under a light source to observe the characteristic shimmering, milky, or bluish sheen that appears to float within the stone. This effect is best seen on smooth, rounded surfaces.
Check for Cleavage
Feldspars exhibit good cleavage in two directions. Look for flat, reflective surfaces where the mineral has broken along these planes. The angles between these cleavage planes can help distinguish between orthoclase (near 90°) and plagioclase (slightly less than 90°).
Consider Hardness
Moonstone has a Mohs hardness of 6-6.5. While not a definitive field test, it can help rule out much softer or harder minerals. It can be scratched by quartz but will scratch glass.
Examine Crystal Habit
While often massive, moonstone can form prismatic or tabular crystals. Look for these forms in vugs or pockets within host rocks.
Similar Rocks
Labradorite
Plagioclase Feldspar (Labradorite variety)
Also known as: Spectrolite (a high-quality variety)
Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Also known as: Precious Opal, Common Opal
Chalcedony
Cryptocrystalline Quartz (SiO2)
Also known as: Agate, Jasper, Carnelian
Scientific Classification
- Mineral Class
- Silicates
- Group
- Feldspar Group (specifically Alkali Feldspar Subgroup for Orthoclase, and Plagioclase Feldspar Subgroup for Albite-Oligoclase)
- Crystal System
- Monoclinic (Orthoclase/Adularia); Triclinic (Plagioclase/Peristerite)
- Chemical Formula
- KAlSi3O8 (Orthoclase/Adularia); (Na,Ca)Al(Si,Al)Si2O8 (Plagioclase/Peristerite)
- Composition
- Potassium aluminum silicate (for orthoclase moonstone); Solid solution series of sodium aluminum silicate (albite) and calcium aluminum silicate (anorthite) for plagioclase moonstone.
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