Nickel-iron meteorites

Nickel-iron meteorites (often called iron meteorites) are one of the three main groups of meteorites. The nickel-iron group has an approximate composition ratio of more than 80 percent metals to less than 20 percent stony material. Most of the metal in them is iron, with nickel in much smaller amounts.

Overview

Meteorites are objects of extraterrestrial origin that have intersected the orbit of the Earth, survived passage through the atmosphere, and reached the Earth’s surface in various stages of preservation. Mineralogically, meteorites may contain various proportions of nickel-iron alloys, silicates, sulfides, and various other minor minerals. They are broadly classified into three major groups: nickel-iron meteorites (often called iron meteorites or just irons), stony-iron meteorites (or stony-irons), and stony meteorites (or stones). This classification is based on the ratio of metallic to stony minerals. The irons generally contain about 80 percent metals, the stony-irons have about a 50-50 ratio, and the stones generally contain more than 80 percent stony minerals.

Stony meteorites are by far the most common, accounting for approximately 95 percent of the meteorites observed to fall to Earth and the large number of meteorites collected in Antarctica. Iron meteorites were once thought to be much more common than they are since most meteorites that had been found (not just those observed falling) were irons. Iron meteorites are more easily noticed on the ground, standing out more distinctly from terrestrial rocks than the other types. Collecting meteorites in Antarctica, where all types stand out prominently against a white background of snow and ice, has shown that irons account for only about 3 to 4 percent of recovered meteorites. Stony irons are rarer, accounting for no more than 1 percent of the total.

Iron meteorites are nickel-iron alloy minerals that occur in the metallic state. There is no native terrestrial equivalent for these minerals, and in fact, the only native metallic iron found on Earth is in small amounts on Disko Island, Greenland, and Josephine, Oregon. The most common form in which terrestrial iron is found is in the oxide state, hematite, magnetite, and limonite minerals. In contrast, the conditions under which meteoritic iron formed were oxygen-poor. This absence of oxygen, combined with the percentage of nickel alloyed with iron, indicates an extraterrestrial origin.

Iron meteorites, or siderites as they were once called, are characterized by the presence of two nickel-iron alloy phases consisting of kamacite (Fe₉₃ Ni₇) and taenite (Fe₆₅ Ni₃₅) combined with minor amounts of troilite (FeS) and other rare mineral phases. Based on the percentage of nickel to iron present, iron meteorites are divided into three subgroups: hexahedrites, octahedrites, and ataxites. Hexahedrites possess a bulk chemical composition of 4-6 percent nickel, occurring principally in large single crystals of the mineral kamacite. Octahedrites, the most common, contain increasing amounts of nickel, appearing in taenite and kamacite mineral forms. The third group, the ataxites, has a nickel content in excess of 18 percent, with taenite and an intergrowth mixture of kamacite and taenite called plessite present.

The two nickel-iron alloy minerals, kamacite (up to 7.5 mass percent nickel) and taenite (between 20 and 50 mass percent nickel) are the two most abundant minerals in iron meteorites. More than forty other minerals have also been identified but are present in only minor amounts. Among these minerals, troilite, diamond, and graphite are the most significant. The others have no terrestrial equivalent and have been reported only from meteorite studies.

The mineralogy of iron meteorites is also unique in textural appearance due to the relationship between the coexisting kamacite and taenite during the meteorite’s cooling process. A mixture of kamacite and taenite produces a geometric pattern of intersecting crystals called Widmanstätten structure, named for its discoverer, Alois Josep Widmanstätten (1754-1849), director of the Imperial Porcelain Works in Vienna. This weave-like or crosshatched pattern is revealed when a cut surface of the iron meteorite is polished and then etched with nitric acid. The pattern results from plates of kamacite occurring in octahedral orientation, with the spaces between filled with taenite. The pattern's bandwidth depends on the kamacite plates' width, which varies according to their nickel content. The pattern is thought to result from slow cooling over millions of years while the iron resided inside a small asteroid-sized body. The Widmanstätten pattern does not occur in any known terrestrial rock. It is an essential criterion in positively identifying a piece of iron as an iron meteorite

The formation of the three subgroups of iron meteorites is directly related to the amount of nickel initially present, falling temperatures, and the resulting rearrangement of iron and nickel atoms; each subgroup was produced as a certain temperature was passed. The process began as the temperature fell below about 1,700 kelvins (1,400° Celsius), allowing taenite to crystallize. When the temperature dropped below about 1,120 kelvins (850° Celsius), diffusion of nickel occurred, and the crystal structure of the taenite readjusted to accommodate the formation of kamacite. That was possible because both minerals have crystal structures with cubic symmetry, but the size difference between nickel and iron atoms gives each mineral a different crystal form. Kamacite has a “body-centered” crystal lattice; each atom is found at a cube's center and surrounded by eight neighboring atoms. In contrast, taenite has a “face-centered” crystal lattice, with an atom centered on each face of a cube; each atom is surrounded by twelve neighboring atoms. The packing arrangement of the atoms in taenite is more compact, thus allowing it to fill the spaces between the kamacite plates.

The study of cooling rates as determined for numerous iron meteorites reveals a wide range. This finding implies that they originated at several depths rather than in a single core, as once thought. If so, the parent body would have been relatively small (probably between 100 and 300 kilometers in diameter) and had insufficient mass to melt its interior. Partial melting could have occurred due to radioactive heating as isotopes such as aluminum 26 decayed; this could have created pockets of molten nickel-iron randomly scattered throughout the parent body. Later impacts with similar-sized bodies could have freed them to assume independent orbits as relatively pure lumps of metal alloys. The shock deformation lamellae (called Neumann lines) seen in the hexahedrites may be evidence of such events.

Methods of Study

Field recognition of a meteorite is not easy unless one is very familiar with its distinctive characteristics. Usually, the most apparent feature of a meteorite will be its unusual heaviness compared to terrestrial rocks of similar size. This is especially the case for iron meteorites since they are generally about three times denser than typical Earth rocks. Another easily testable property of an iron meteorite is its strong attraction to a magnet. The surface of a meteorite is relatively smooth and featureless but will often exhibit flowlines, furrows, shallow depressions, and deep cavities. One characteristic surface feature is shallow depressions known as thumbprints because they resemble the imprints of thumbs pressed into soft clay. Newly fallen meteorites can also exhibit a fusion crust, which shows the effects of intense atmospheric heating upon its surface. This crust resembles black ash, but it will weather to a rusty brown and even disappear with time. The fusion crust on iron meteorites is not particularly distinctive and does weather rapidly.

In most cases, positive confirmation of an iron meteorite must be made in the laboratory. A small corner of the specimen can be cut, polished, and etched with acid to look for Widmanstätten patterns. If these patterns are found, the specimen is an iron meteorite, but not all iron meteorites show Widmanstätten structure. A relatively simple chemical test for the presence of nickel can be made by dissolving a small amount of the specimen in hydrochloric acid; then tartaric acid, 1 percent solution of dimethylglyoxime in ethanol, and ammonium hydroxide are added. If the solution contains nickel, a scarlet precipitate will result. A quantitative analysis is then conducted to determine the nickel's actual mass percentage. This determination will confirm the sample's identity because nickel content in meteorites falls within a very specific range.

Over the years, various criteria have been used to classify iron meteorites. Some more obvious have been chemical, structural, and mineralogical; others include cosmic-ray exposure ages and cooling rates. A widely used system, which goes back to the late 1800s, is based on the bandwidth of the Widmanstätten structure (the octahedral array of kamacite) as seen on a cut, polished, and etched surface. The width of these kamacite bands depends on nickel content and cooling rates. Bulk nickel content generally increases as the bandwidth of kamacite decreases, thus providing a criterion for assigning individual specimens to common groups. Chemical studies for trace elements have extended this classification scheme by including analyses for gallium and germanium. A good correlation has been found between bandwidth size and gallium content, thus permitting a finer separation of iron meteorites into smaller subgroups.

Studies that classify the irons into specific types also provide clues to the meteorite’s origin and the nature of its parent body. Estimating the cooling rate for the coexisting kamacite and taenite can provide evidence of conditions at the time of the meteorite’s origin. This cooling rate has been determined from crystallization experiments in the laboratory and direct observation of the mineral phases found in iron meteorites. The estimated cooling rates vary with the kamacite phase's bandwidth sizes, which provides a correlation between cooling rates and bulk chemical composition.

The determination of the cosmic-ray exposure age of a meteorite indicates when the object broke out of its parent body. This technique may also lead to matching individual meteorite specimens to a common event. In addition, the compositions and abundances of minor and trace minerals, along with the extent of shock damage to their structures, might give a clearer picture of the events that led to and occurred during the parent body’s breakup. Studies such as these reveal clues not only about the origin of the meteorite but also about the formation of the Earth.

Context

Nickel-iron meteorites affected early human history and technology. Some of the earliest historical records from ancient Egypt speak of iron falling from the sky, and it was undoubtedly meteoritic iron that was first fashioned into iron tools and weapons. Studies have shown that iron tools manufactured on the South Pacific Islands, where no local source of iron could be found, were forged from meteoritic iron. Some ancient cultures also worshiped “heavenly” iron and placed it in the burial tombs of their leaders; it was thought to be a gift from the gods and symbolized wealth and power. Iron became more valuable than gold in Europe as the Bronze Age ended. Perhaps in the not-too-distant future, space colonists will be mining iron asteroids to provide for their industrial needs.

In the twenty-first century, nickel-iron meteorites are helping reveal planetary formation processes in the early solar system. They provide evidence of what the interiors of the terrestrial planets may be like. The Earth’s core is probably composed of a nickel-iron alloy similar to that of iron meteorites.

The scars of giant impacts, many due to iron meteorites, dot the Earth’s surface from Arizona to Australia. Perhaps the most recent testimony to the effects of a giant meteorite impact can be seen at Meteor Crater near Winslow, Arizona. More than twenty thousand years ago, an iron meteorite weighing more than 100,000 tons collided with the Earth at that site. The resulting crater, which measures more than one kilometer across, and nearly 200 meters deep, was created by an object about thirty meters across traveling at fifteen kilometers per second. The energy released at impact was on the order of a two to three-megaton nuclear weapon, destroying most of the meteorite in the process, but that it was an iron meteorite has been confirmed because broken fragments and solidified droplets of meteoritic iron have been recovered around the site.

The largest known intact meteorite is an iron meteorite, the Hoba West meteorite, which weighs an estimated sixty tons and is still embedded in the ground where it fell near Grootfontein, Namibia in southwestern Africa. The largest meteorite on display in a museum is another iron meteorite, the Ahnighito meteorite, which weighs 34 tons; it was found by the arctic explorer R. E. Perry near Cape York, Greenland, in 1894, and brought to New York for display in the American Museum of Natural History.

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