Olivine is a magnesium iron silicate mineral commonly found in mafic and ultramafic igneous rocks. Its color ranges from olive green to yellow-green, and it often exhibits a glassy luster.
Olivine is typically composed of two end-members: forsterite (Mg2SiO4) and fayalite (Fe2SiO4), forming a solid solution series between these two extremes. This mineral is noted for its high-temperature stability and its role as a primary constituent in the Earth’s mantle.
Formation and Occurrence
Olivine forms in high-temperature environments, typically within the Earth’s mantle, and is brought to the surface through volcanic activity. It is a major component of peridotite and dunite and can also be found in basalt and gabbro. Olivine is often present in xenoliths, fragments of mantle rock brought up by volcanic eruptions, providing valuable insights into mantle composition and processes.
Physical Properties
- Chemical Formula: (Mg,Fe)2SiO4
- Crystal System: Orthorhombic
- Hardness: 6.5–7 on the Mohs scale
- Specific Gravity: 3.2–4.3 (varies with iron content)
- Cleavage: Poor, typically along {010} and {100}
- Fracture: Conchoidal to uneven
- Luster: Vitreous (glassy)
- Transparency: Transparent to translucent
Varieties and Colors
Olivine ranges in color from pale yellow to olive green, depending on the iron content. The more magnesium-rich forsterite is typically lighter, while the iron-rich fayalite tends to be darker. Gem-quality olivine is known as peridot and is prized for its bright green hue and is often used in jewelry.
Notable Locations
- Hawaii: Basaltic lava flows containing olivine crystals
- Arizona, USA: Peridot Mesa, known for gem-quality peridot
- Myanmar (Burma): Source of high-quality peridot
- Norway: Peridotite and dunite deposits rich in olivine
- Pakistan: Gem-quality peridot found in the Kohistan region
Uses and Significance
Olivine is primarily used as a gemstone (peridot) and as an industrial mineral. In its gem form, peridot has been valued since ancient times for its distinctive green color. Industrially, olivine is used in refractory materials due to its high melting point and in sandblasting and casting sands. Additionally, olivine plays a critical role in understanding geological processes, particularly mantle dynamics and magmatic differentiation.
Care and Maintenance
Olivine, though relatively hard, can be sensitive to sudden temperature changes and acid exposure. It should be cleaned with mild soap and warm water, avoiding harsh chemicals and extreme temperature variations. When used in jewelry, olivine should be stored separately to prevent scratching by harder materials.
Identification Tips
- Color: Olive green to yellow-green, often distinctive
- Hardness: 6.5–7 on the Mohs scale, harder than glass
- Streak: White or colorless
- Luster: Vitreous, glassy appearance
- Crystal Habit: Typically granular or massive, sometimes forming short prismatic crystals
Fun Facts About Olivine
- Space Traveler: Olivine has been found in meteorites, providing information about the early solar system.
- Green Sand Beaches: Some beaches, like Papakolea Beach in Hawaii, have green sand composed largely of olivine crystals.
- Ancient Symbolism: Peridot, the gem variety of olivine, was believed to protect its wearer from nightmares and bring happiness and prosperity.
- Extraterrestrial Olivine: The mineral has also been identified on the Moon and Mars.
- Birthstone: Peridot is the birthstone for August and is associated with the zodiac sign Leo.
- Lava Lamps: Naturally occurring olivine can be seen embedded in some types of volcanic lava flows.
- Gemstone History: Ancient Egyptians called peridot the “gem of the sun” and it was used in many of their artifacts.
- Olivine in the Ocean: Small grains of olivine can be found in oceanic sediments, often carried there by rivers and streams from volcanic sources.
