A meteorite is a natural object that originates in space, survives its passage through the atmosphere, and lands on Earth’s surface. These extraterrestrial samples represent the oldest, most pristine material available for study, predating the formation of planets. Their composition provides a physical record of the earliest materials that existed in the solar nebula over 4.5 billion years ago, helping scientists understand the processes that built our solar system.
The Fundamental Building Blocks
The elemental composition of meteorites is dominated by the same elements found on Earth, but their arrangement and proportions are markedly different. The most abundant components are silicates, minerals primarily composed of silicon and oxygen. Magnesium-rich olivine and pyroxene are the most common minerals, forming the bulk of the rocky material.
Beyond the silicates, a significant portion of meteorites contains metallic components, mainly an alloy of iron and nickel. This iron-nickel metal is often present as small, scattered grains throughout the rock, or it can be the primary material itself. The presence of iron-nickel metal in a free state is what distinguishes space rocks from most terrestrial material. Meteorites also contain sulfides, most commonly troilite, an iron sulfide mineral rarely found in Earth’s crustal rocks.
Primary Classification by Composition
Meteorites are broadly grouped into three major classes based on their bulk composition and origin.
Iron Meteorites
Iron meteorites consist almost entirely of a dense iron-nickel metal alloy. These samples are fragments of the metallic cores of ancient, differentiated asteroids shattered by collisions. Their metallic nature results in an extremely high density, making them notably heavy.
Stony-Iron Meteorites
Stony-Iron meteorites are the rarest class, containing roughly equal proportions of silicate minerals and iron-nickel metal alloy. This mixed composition often creates a striking appearance, such as in pallasites, where large olivine crystals are suspended within a metal matrix. They likely originated from the boundary layer between the metallic core and the silicate mantle of their parent asteroids.
Stony Meteorites
Stony meteorites form the largest class, making up over 95% of all known falls. They are composed predominantly of silicate minerals, similar to Earth’s mantle and crust. Even these silicate-rich samples often contain small, scattered grains of iron-nickel metal, a feature rarely seen in terrestrial rocks.
Detailed Composition of Stony Meteorites
Stony meteorites are subdivided into Chondrites and Achondrites, based on whether the rock has been melted and chemically processed.
Chondrites
Chondrites are the most primitive and common type, defined by the presence of chondrules. Chondrules are small, spherical mineral grains, mainly composed of olivine and pyroxene, that represent droplets of silicate melt rapidly cooled in the early solar nebula. Since the material in chondrites has not been fully melted or differentiated, they preserve the original chemical composition of the solar system’s dust and gas cloud.
Achondrites
Achondrites lack chondrules and have a texture similar to volcanic rocks on Earth. Their composition indicates they originated from a parent body that underwent differentiation, a process where heat caused the body to melt and separate into layers by density. The resulting achondrite rocks are igneous, meaning they crystallized from a molten state, which erased the primitive chondritic structure. Achondrites still provide valuable clues about the internal structure and evolution of small planetary bodies.
Distinguishing Meteoritic Material from Earth Rocks
The unique composition of meteorites provides several practical ways to distinguish them from common terrestrial rocks. The presence of iron-nickel metal is a reliable chemical indicator, especially since the metal phase in meteorites is characteristically high in nickel, typically containing between 5% and 35% nickel. This high metal content often makes the meteorite strongly magnetic and gives iron and stony-iron types an unusually high density compared to similarly sized Earth rocks.
Another key compositional difference is the overall mineralogy. Most Earth rocks contain quartz or other silica-rich minerals, but meteorites do not contain a significant amount of these compounds. Furthermore, fresh meteorites lack oxidized iron, or rust, containing iron instead in its metallic or ferrous state. The intense heat generated during atmospheric entry results in the formation of a thin, dark, glassy layer on the surface called a fusion crust. This crust is a compositional alteration of the surface material, which quickly melted and resolidified, providing a visual identifier of the rock’s extraterrestrial journey.