A meteorite is a piece of rock or metal that originated in space and survived passage through Earth’s atmosphere to land on the surface. Meteorites are fundamentally crystalline materials, meaning their internal structure is an organized arrangement of atoms. These structures formed over billions of years in the solar system’s earliest environments. Studying these extraterrestrial crystals provides scientists with tangible evidence of the materials and processes that built our solar system.
The Basic Mineral Components of Meteorites
The crystalline components of meteorites fall into two major groups: silicate minerals and iron-nickel alloys. The most common silicate crystals are olivine and pyroxene, which form the bulk of stony meteorites. Olivine is a magnesium-iron silicate, while pyroxene crystals are silicates that exhibit a highly ordered atomic structure.
In metallic meteorites, the crystalline structure is composed of iron-nickel alloys. The two primary metal crystals are kamacite, which contains a low percentage of nickel (up to 7.5%), and taenite, which has a much higher nickel content (more than 25%). The proportion of these specific compounds allows scientists to classify and determine the meteorite’s origin.
Crystallization Processes in the Early Solar System
The formation of crystals began with the cooling and condensation of the solar nebula, the cloud of gas and dust from which our solar system formed. The first solids were tiny, high-temperature minerals that condensed directly from the hot gas phase at temperatures over 1,000 degrees Celsius, creating the initial crystalline building blocks.
Differentiation and Slow Cooling
As planetesimals accreted, internal heating from radioactive isotopes caused some to melt and differentiate, separating into layers. Denser materials like iron sank to form a core, leaving the lighter silicate minerals to crystallize in a mantle and crust. The extremely slow cooling of these metallic cores, sometimes as slow as 100 to 10,000 degrees Celsius per million years, was necessary for the formation of large, distinct crystal structures.
Shock Metamorphism
Existing crystal structures were also altered by high-energy impacts between celestial bodies. These shock events caused localized melting, high-pressure phase changes, and physical deformation of the crystal lattice. For example, the mineral coesite, a dense form of quartz, forms only under the immense pressures of a major impact. The overall process of crystallization in space, therefore, involved everything from initial gas-phase condensation to slow-motion cooling and violent shock metamorphism.
Crystalline Signatures of Major Meteorite Classes
The three major classes of meteorites—stony, iron, and stony-iron—are defined by their distinct crystalline signatures.
Stony Meteorites (Chondrites)
The most common meteorites, the stony chondrites, are characterized by chondrules. Chondrules are millimeter-sized, spherical droplets of silicate minerals, primarily olivine and pyroxene, that flash-melted and rapidly crystallized while free-floating in the solar nebula. These tiny spheres are the oldest solid materials in the solar system, providing a unique snapshot of its birth.
Iron Meteorites
Iron meteorites display a striking, geometric pattern called the Widmanstätten structure when cut, polished, and etched with acid. This distinctive pattern is an interlocking lattice of the two iron-nickel alloys, kamacite and taenite. The structure requires cooling that takes millions of years, allowing the two metal phases to slowly separate and grow into large, intersecting crystal plates. The presence of the Widmanstätten pattern serves as proof of a meteorite’s cosmic origin.
Stony-Iron Meteorites (Pallasites)
Stony-iron meteorites, known as pallasites, represent the boundary layer between the metal core and the silicate mantle of an ancient planetesimal. Their crystalline signature is visually stunning, consisting of large, often olive-green crystals of olivine embedded within a continuous matrix of iron-nickel metal. These crystals provide direct evidence of the internal structure and differentiation of their parent bodies.
Reading the Cosmic History Locked in Crystals
Crystals within meteorites act as precise chronometers and environmental sensors. By analyzing the decay of radioactive isotopes, scientists can determine the crystallization age of the meteorite materials. This analysis has dated the solar system’s earliest solids to approximately 4.56 billion years ago, establishing a timeline for planet formation.
The composition and structure of these crystals reveal the temperature and pressure conditions under which they formed. The size of the crystals in the Widmanstätten pattern provides an accurate measure of the cooling rate of the parent asteroid’s core. Furthermore, variations in isotopic signatures preserved in minerals have confirmed a chemical divide in the early solar nebula. This suggests that material from the inner solar system remained largely separate from that of the outer solar system during accretion.