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Chondrites are the most common type of meteorite, accounting for over 80% of all meteorites that fall to Earth. They are characterized by the presence of chondrules, which are small, spherical to sub-spherical mineral grains, typically silicates, that formed as molten droplets in the solar nebula. Chondrites are considered primitive, undifferentiated material, meaning they have not undergone significant melting or geological processing since their formation. They contain a mixture of silicates (olivine, pyroxene), metal (iron-nickel alloy), and sulfides, often with a fine-grained matrix. Their composition reflects the bulk composition of the solar nebula, excluding volatile elements.
How to Identify
- Color
- Typically dark grey, black, or brownish-black, often with a rusty patina due to terrestrial weathering of iron-nickel metal.
- Luster
- Dull to sub-metallic on fresh surfaces, often with small, shiny flecks of metal.
- Texture
- Fine-grained matrix with embedded chondrules (millimeter-sized spherical inclusions) and metallic grains. Fusion crust, if present, is smooth and often black.
- Crystal Form
- Chondrules are typically spherical to sub-spherical. Individual mineral grains within the matrix are anhedral to subhedral. No macroscopic crystal form for the meteorite as a whole.
- Cleavage
- No distinct cleavage for the bulk rock. Individual mineral components (olivine, pyroxene) may exhibit cleavage.
- Geological Environment
- Chondrites are not formed in terrestrial geological environments. They are found on Earth's surface after falling from space, often in arid regions (deserts) or polar ice fields where they are preserved and concentrated.
Key Facts
- Hardness: Variable, typically 5-6.5 on Mohs scale (due to silicate minerals and metal)
- Specific Gravity: 3.3 - 3.8 g/cm3 (higher than most terrestrial rocks due to metal content)
- Crystal System: Not applicable for the bulk rock; individual minerals (olivine, pyroxene) are orthorhombic or monoclinic; metal is isometric.
- Color: Dark grey, black, brownish-black, often with rusty patches
- Luster: Dull to sub-metallic, metallic flecks
- Transparency: Opaque
- Fracture: Irregular to conchoidal
- Cleavage: None for the bulk rock; individual minerals may show cleavage.
- Composition: Silicates (olivine, pyroxene), iron-nickel metal (Fe-Ni alloy), troilite (FeS), and minor accessory minerals.
Quick Check
- Color: Dark grey to black, often rusty brown
- Luster: Dull to sub-metallic
- Streak: Grey or no streak
Physical Characteristics
- Crystal Habit: Aggregates of fine-grained matrix, chondrules, and metallic grains. No macroscopic crystal habit for the meteorite itself.
- Cleavage Type: Not applicable for the bulk rock. Individual mineral components may have distinct cleavage (e.g., pyroxene).
- Fracture Type: Irregular to sub-conchoidal.
- Tenacity: Brittle.
- Luster Type: Dull to sub-metallic, with metallic flecks.
Formation
Chondrites are formed from the accretion of dust and small grains in the early solar nebula. They are largely unaltered remnants of the material that formed the planets, having undergone little to no melting or differentiation since their formation. The characteristic chondrules are millimeter-sized spherical or sub-spherical silicate-rich inclusions that formed as molten droplets in the solar nebula and subsequently cooled and crystallized.
Usage
Chondrites are primarily of scientific interest, providing invaluable insights into the early solar system, planetary formation, and the composition of primitive solar nebula material. They are also highly prized by collectors and museums.
Age Distribution
Approximately 4.56 billion years old (coeval with the formation of the solar system)
Where to Find
Antarctica
Large numbers of meteorites are found on the blue ice fields of Antarctica, where glacial movement concentrates them and the dry, cold conditions preserve them well.
Sahara Desert
The vast, arid plains of the Sahara Desert are excellent meteorite hunting grounds, as the dark meteorites stand out against the light-colored sand and are well-preserved in the dry climate.
Atacama Desert, Chile
Similar to the Sahara, the extreme aridity of the Atacama Desert leads to excellent preservation and accumulation of meteorites.
Nullarbor Plain, Australia
Another large, arid region where meteorites are found due to good preservation and visibility.
Finding Tips
Magnetism Test
Most chondrites contain iron-nickel metal and are therefore attracted to a strong magnet. This is a primary distinguishing feature from most terrestrial rocks.
Density Test
Chondrites are generally denser than common terrestrial rocks of similar size due to their metallic content. They will feel 'heavier' than expected.
Fusion Crust
Freshly fallen meteorites often have a thin, dark, glassy fusion crust formed by melting during atmospheric entry. This crust may be abraded or weathered on older finds.
Thumbprint-like Indentations (Regmaglypts)
Some meteorites, particularly larger ones, may exhibit shallow, thumbprint-like indentations on their surface, formed by ablation during atmospheric passage.
Chondrules (Internal Feature)
If a small piece is broken off or a surface is polished, the presence of small, spherical chondrules is diagnostic of a chondrite. A magnifying glass or hand lens is often needed.
Streak Test (Caution)
While not always definitive, a fresh surface of a chondrite will typically produce a grey or no streak on unglazed porcelain, unlike hematite (red-brown streak) or magnetite (black streak).
Avoid Contamination
When collecting, handle meteorites as little as possible and store them in clean, dry, inert containers (e.g., zip-lock bags) to prevent terrestrial contamination and weathering.
Similar Rocks
Achondrite Meteorite
Achondrite
Also known as: Differentiated Meteorite
Iron Meteorite
Iron Meteorite
Also known as: Metallic Meteorite
Stony-Iron Meteorite
Stony-Iron Meteorite
Also known as: Pallasite, Mesosiderite
Terrestrial Basalt
Basalt
Also known as: Volcanic Rock
Hematite
Fe2O3
Also known as: Kidney Ore
Scientific Classification
- Mineral Class
- Not a single mineral; a rock composed of various minerals and metal.
- Group
- Stony Meteorite (Chondrite Group)
- Crystal System
- Not applicable for the bulk rock.
- Chemical Formula
- Variable, complex mixture of silicates, Fe-Ni metal, and sulfides.
- Composition
- Primarily olivine ((Mg,Fe)2SiO4), pyroxene ((Mg,Fe)SiO3), iron-nickel alloy (Fe-Ni), and troilite (FeS). Minor components include plagioclase, chromite, and phosphates. The exact proportions vary depending on the chondrite group (e.g., H, L, LL, CM, CV, CO, CR, etc.).
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