Iron Meteorite
Introduction
Iron meteorites are a type of meteorite that are primarily composed of iron and nickel. These extraterrestrial objects are remnants of the cores of ancient planetary bodies that have since been destroyed. Iron meteorites are among the most scientifically significant and visually striking meteorites, providing valuable insights into the formation and differentiation of planetary bodies in the early solar system.
Composition and Classification
Iron meteorites are predominantly composed of iron (Fe) and nickel (Ni), with trace amounts of other elements such as cobalt (Co), phosphorus (P), and sulfur (S). They are classified based on their structural and chemical properties into three main groups: hexahedrites, octahedrites, and ataxites.
Hexahedrites
Hexahedrites are composed almost entirely of kamacite, a nickel-iron alloy with a low nickel content (typically 5-7%). They lack the Widmanstätten pattern, a distinctive intergrowth of kamacite and taenite that is visible when the meteorite is etched with acid.
Octahedrites
Octahedrites are the most common type of iron meteorite and contain both kamacite and taenite. They exhibit the Widmanstätten pattern, which is a result of the slow cooling of the metal, allowing the formation of interlocking bands of kamacite and taenite. Octahedrites are further subdivided based on the width of these bands into coarse, medium, and fine octahedrites.
Ataxites
Ataxites are rare iron meteorites with a high nickel content (typically above 16%). They lack the Widmanstätten pattern and are composed primarily of taenite. The high nickel content gives ataxites a more homogeneous structure compared to hexahedrites and octahedrites.
Formation and Origin
Iron meteorites are believed to originate from the cores of differentiated parent bodies, such as asteroids or protoplanets, that underwent melting and differentiation early in the solar system's history. These parent bodies experienced significant heating, causing the heavier elements like iron and nickel to sink to the core, while lighter silicate minerals formed the mantle and crust.
When these parent bodies were subsequently shattered by collisions with other celestial objects, fragments of their metallic cores were ejected into space. Some of these fragments eventually entered Earth's atmosphere and fell as meteorites.
Widmanstätten Pattern
The Widmanstätten pattern is a unique intergrowth of kamacite and taenite that is characteristic of many iron meteorites, particularly octahedrites. This pattern is revealed when a polished surface of the meteorite is etched with a weak acid solution, such as nitric acid.
The formation of the Widmanstätten pattern is a result of the slow cooling rates experienced by the parent bodies of iron meteorites, typically on the order of 1-100°C per million years. This slow cooling allowed the diffusion of nickel and the growth of kamacite crystals within the taenite matrix.
Chemical and Isotopic Analysis
Chemical and isotopic analyses of iron meteorites provide valuable information about the conditions and processes that occurred in the early solar system. Techniques such as mass spectrometry are used to determine the concentrations of various elements and isotopes within the meteorite.
One important aspect of these analyses is the study of isotopic anomalies, which can reveal information about the nucleosynthetic processes that occurred in the stellar environments where the meteorite's constituents were formed. For example, variations in the isotopic ratios of elements such as chromium (Cr) and titanium (Ti) can provide insights into the types of supernovae that contributed to the solar system's material.
Tetrataenite and Other Rare Phases
In addition to the common phases of kamacite and taenite, iron meteorites can contain rare phases such as tetrataenite, a highly ordered form of taenite with a specific nickel content. Tetrataenite forms over long periods of time and requires extremely slow cooling rates, making it an indicator of the thermal history of the meteorite.
Other rare phases found in iron meteorites include schreibersite, a nickel-iron phosphide, and cohenite, an iron carbide. These phases can provide additional information about the conditions within the parent body and the processes that led to the formation of the meteorite.
Meteorite Showers and Strewn Fields
When a large iron meteorite enters Earth's atmosphere, it can break apart due to the intense heat and pressure, resulting in a meteorite shower. The fragments from such an event can be spread over a large area, known as a strewn field.
Strewn fields are important for meteorite hunters and researchers, as they provide clues about the trajectory and breakup process of the meteorite. Some well-known strewn fields include the Sikhote-Alin in Russia and the Campo del Cielo in Argentina.
Historical and Cultural Significance
Iron meteorites have been known and used by humans for thousands of years. In ancient times, they were a valuable source of metal for tools and weapons, particularly in regions where terrestrial iron ores were scarce. The earliest known use of iron meteorites dates back to the Bronze Age, with artifacts such as the iron beads from Gerzeh, Egypt, which are believed to be made from meteoritic iron.
In addition to their practical uses, iron meteorites have held cultural and symbolic significance in various societies. For example, the Inuit people of Greenland used iron meteorites for toolmaking and regarded them as gifts from the gods. The Cape York meteorite, one of the largest iron meteorites ever discovered, was an important resource for the Inuit for centuries.
Modern Scientific Research
Modern scientific research on iron meteorites involves a multidisciplinary approach, combining techniques from fields such as geochemistry, petrology, and planetary science. Researchers study iron meteorites to gain insights into the processes that shaped the early solar system, the formation and differentiation of planetary bodies, and the history of meteorite impacts on Earth.
One area of active research is the study of cosmic ray exposure ages, which can provide information about the length of time a meteorite has been exposed to cosmic rays in space. This data can help reconstruct the history of the meteorite's journey from its parent body to its arrival on Earth.