Olivine: A Mineralogical Marvel
An in-depth exploration of Olivine, a vital mineral in Earth's upper mantle, detailing its chemical composition, crystal structure, geological occurrences, and diverse applications.
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Mineral Overview
Composition and Classification
Olivine is a magnesium iron silicate mineral with the chemical formula (Mg,Fe)2SiO4. It belongs to the nesosilicate (or orthosilicate) group, characterized by isolated silicate tetrahedra (SiO4). As a fundamental component of Earth's upper mantle, olivine is a ubiquitous mineral in the subsurface, though it weathers rapidly on the surface.[1]
Forsterite and Fayalite
The olivine series represents a solid solution between two endmembers: forsterite (Mg2SiO4) and fayalite (Fe2SiO4). The ratio of magnesium to iron determines its properties. Forsterite exhibits a significantly higher melting temperature (around 1,900 ยฐC at atmospheric pressure) compared to fayalite (around 1,200 ยฐC). This ratio, often expressed as molar percentages (e.g., Fo70Fa30), influences various physical characteristics.[2][3]
Geological Significance
Olivine and its high-pressure polymorphs constitute over 50% of Earth's upper mantle. Its abundance and properties critically influence the rheology of the mantle, driving plate tectonics. It is commonly found in mafic and ultramafic igneous rocks like gabbro and basalt, and in metamorphic rocks derived from impure dolomites.[9][14]
Identification and Properties
Visual and Physical Characteristics
Olivine is typically recognized by its olive-green color, which may deepen to a brownish or reddish hue due to iron oxidation. Gem-quality, translucent olivine is known as peridot. Its crystal habit is often massive or granular. It exhibits poor cleavage, a conchoidal fracture, and a brittle tenacity. The luster is vitreous, and the streak is colorless or white.[6][7]
Physical Measurements
Key physical properties include:
Classification Details
Olivine is classified as a nesosilicate and belongs to the olivine group. Its crystal system is orthorhombic, with the space group Pbnm (no. 62). The IMA symbol is 'Ol', and its Strunz classification is 9.AC.05.[1]
Extraterrestrial Occurrences
Beyond Earth
Olivine's presence extends beyond our planet. It has been identified in meteorites, including chondrites and pallasites. Furthermore, olivine has been detected on the Moon and Mars, and its spectral signature is observed in dust disks surrounding young stars and on asteroids like 25143 Itokawa. Comet dust samples have also confirmed the presence of olivine.[19][20][21][22][23][24][25][26][27]
Cosmic Significance
The detection of olivine in extraterrestrial environments provides crucial insights into the composition and evolution of planetary bodies and protoplanetary disks. Its presence in comets and around young stars suggests that the building blocks of terrestrial planets, including essential minerals, are common throughout the cosmos.
Crystal Structure
Atomic Arrangement
Olivine crystallizes in the orthorhombic system (space group Pbnm), featuring isolated silicate tetrahedra (SiO4). This structure can be visualized as a hexagonal close-packed array of oxygen ions, with magnesium or iron ions occupying half the octahedral sites and silicon ions occupying one-eighth of the tetrahedral sites. The structure incorporates distinct oxygen, metal (M1, M2), and silicon sites, with specific symmetry elements influencing the atomic positions.[11]
Microscopic View
Under a microscope, particularly in cross-polarized light, olivine crystals exhibit characteristic interference colors and optical properties. The atomic structure, when viewed along the crystallographic axes, reveals the precise arrangement of oxygen, silicon, and metal ions, forming the framework of this essential mineral.
High-Pressure Polymorphs
Mantle Transitions
At the extreme pressures and temperatures found deep within Earth's mantle, the olivine structure becomes unstable. Below approximately 410 km depth, olivine undergoes phase transitions. It transforms into wadsleyite (a sorosilicate) and subsequently into ringwoodite (a spinel structure) at greater depths. This transition is exothermic, influencing mantle convection dynamics.[28]
Ringwoodite Decomposition
At around 660 km depth, ringwoodite decomposes into silicate perovskite and ferropericlase. This reaction is endothermic and creates a significant density increase, observable via seismic methods. These phase transitions are crucial for understanding mantle geophysics and the Earth's internal structure.[28]
Surface Weathering
Instability on the Surface
Olivine is relatively unstable on Earth's surface and weathers rapidly in the presence of water. It commonly alters into iddingsite, a mixture of clay minerals, iron oxides, and ferrihydrite. This rapid alteration means olivine is rarely preserved in sedimentary rocks.[30][34]
Carbon Sequestration Potential
The rapid weathering of olivine has led to proposals for its use in carbon sequestration. Dispersing finely ground olivine, particularly on beaches where wave action increases surface area and agitation, could accelerate the reaction with atmospheric CO2 to form stable carbonates.[31][32][44][45][46]
Mining and Production
Norway's Dominance
Norway is a primary global source for industrial-grade olivine, particularly in the Sunnmรธre and Nordfjord regions. Open-pit mines, such as the one at Robbervika in Norddal municipality, have been operational since the 1980s. Historically, olivine was also used as sharpening stones, with early descriptions noting its surface red color and internal blue hue.[35][36][37][38][39]
Industrial Scale
Approximately 50% of the world's olivine for industrial applications is sourced from Norway. The mining operations extract large quantities, reflecting the mineral's demand in various industrial sectors. Some mining sites are also designated as nature reserves, highlighting the balance between resource extraction and conservation.[40]
Industrial and Gemstone Applications
Foundry and Metallurgy
Olivine sand serves as a substitute for silica sand in the aluminum foundry industry. Its lower water requirement and ability to maintain mold integrity during metal pouring are advantageous. In steelworks, olivine is used as a fluxing agent, replacing dolomite due to its favorable properties.[41][42]
Sauna Stoves
In Finland, olivine is highly valued for its suitability in sauna stoves. Its high density and resistance to thermal shock from repeated heating and cooling cycles make it an ideal material for generating steam and heat.[43]
Experimental Uses
Carbon Capture
Beyond industrial applications, olivine is being investigated for its potential in climate change mitigation. The reaction of crushed olivine with atmospheric carbon dioxide is a slow but potentially effective method for carbon sequestration, yielding stable carbonates.[44][45][46]
Sustainable Cement
Researchers are exploring the use of olivine-derived materials in the production of carbon-neutral or even carbon-negative cement. This innovative approach aims to reduce the environmental impact of the construction industry by utilizing minerals like olivine in cementitious materials.[47]
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References
References
- St. John's Island peridot information and history at Mindat.org
- Strรยธm, Hans: Physisk og Oeconomisk Beskrivelse over Fogderiet Sรยธndmรยธr beliggende i Bergen Stift i Norge. Published in Sorรยธ, Denmark, 1766.
- Mineralressurser i Norgeย ; Mineralstatistikk og bergverksberetning 2006. Trondheim: Bergvesenet med bergmesteren for Svalbard. 2007.
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Important Notice
This content has been generated by an AI and is intended for educational and informational purposes only. While efforts have been made to ensure accuracy based on the provided source material, it may not be exhaustive or entirely up-to-date.
This is not professional geological or material science advice. The information presented here should not substitute for consultation with qualified geologists, mineralogists, or material scientists. Always consult with experts for specific applications or research needs.
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