Types of Meteorites

Meteorites traditionally have been divided into three broad categories—stony meteorites (or stones), iron meteorites (irons), and stony iron meteorites (stony irons)— based on the proportions of rock-forming minerals and nickel-iron (also called iron-nickel) metal alloy they contain. Stony meteorites make up about 94 percent of all known meteorites, irons about 5 percent, and stony irons about 1 percent. There is considerable diversity within each category, leading to numerous subdivisions (classes, groups, etc.) based on variations in chemistry, mineralogy, and structure.

It is important to realize that meteorite classification is based primarily on observable characteristics. Just because subdivisions belong to the same category, it does not necessarily follow that they all consist of meteorites that have the same or similar parent bodies. Indeed, more often than not, they are unrelated. Conversely, subdivisions from different categories may have a common origin. For instance, if a large asteroid were to melt, its denser metallic components would tend to sink to its centre (its core), while its less-dense rocky material would form a mantle around it, much like what happened to Earth. This separation process is known as geochemical differentiation. When the differentiated asteroid is later broken up by collisions, samples of its rocky mantle, iron core, and core-mantle interface might be represented in the three main categories. Thus, the challenge for researchers is to determine which types of meteorites are related and which are not, as well as to identify the processes that were responsible for the tremendous diversity that is seen among them.

Chondrites

The most fundamental distinction between the various stony meteorites is between those that were once molten, the achondrites, and those that were not, the chondrites. Chondrites have been subdivided into three main classes—ordinary, carbonaceous, and enstatite chondrites—and these in turn have been divided into a number of groups.

Chondrites are the most abundant meteorites (about 87 percent of stony meteorites) in collections. They also are arguably the most important. In terms of terrestrial rocks, these meteorites seem akin to sedimentary conglomer-ates—i.e., fragments of preexisting rock cemented together. They are a mechanical mixture of components that formed in the solar nebula or even earlier. Perhaps more remarkably, the compositions of chondrites are very similar to that of the Sun, except for the absence (in chondrites) of very volatile elements such as hydrogen and helium. The Sun contains more than 99 percent of the mass of the solar system. The composition of the Sun must therefore be very close to the average composition of the solar system when it formed. As a result, the Sun's composition can serve as a reference. Deviations in a meteorite's composition from this reference composition provide clues to the processes that influenced the formation of its parent body and the components in it.

Chondrules

Meteorites are classified as chondrites based on the presence within them of small spherical bodies (typically about 1 mm [0.04 inch] in diameter) called chon-drules. From their shapes and the texture of the crystals in them, chondrules appear to have been free-floating molten droplets in the solar nebula. Simulation experiments show that chondrules formed by "flash" heating (to peak temperatures of 1,400-1,800 °C) and then rapid cooling (10-1,000 °C per hour). The sizes, compositions, and proportions of different types of chondrules vary from one chondrite meteorite to the next, which means that chondrule formation must have been a fairly localized process. There is also good evidence for its occurring many times. If chondrule abundance in chondrites is any guide, the chondrule-forming process was one of the most energetic and important in the solar nebula, at least in the region of the asteroid belt. Nevertheless, despite more than a century of study and speculation, scientists have yet to determine definitively what the process was.

Refractory Inclusions

Minor but important constituents of chondrites are refractory inclusions. They are so termed because they are highly enriched in the least-volatile, or refractory, elements. Because calcium and aluminum are two of the most abundant refractory elements in them, they are often called calcium-aluminum-rich inclusions, or CAIs. They range in shape from highly irregular to spherical and in size from tens of micrometres up to a centimetre or more. Like chondrules, they formed at high temperatures but appear to have been heated for more prolonged periods. Many but not all types of inclusions appear to have been formed from a molten state, which probably came about by the heating of preexisting solids. Others seemed to have formed as crystalline solids that condensed directly from a hot gas. Like chondrules, there is no consensus on the mechanism or mechanisms that formed refractory inclusions.

Matrix

The space between the chondrules and refractory inclusions is filled with a finegrained matrix that cements the larger meteoritic components together. The matrix is richer in volatile elements than are chondrules and inclusions, suggesting that at least some fraction of it formed at a lower temperature. The matrix of many chondrites contains organic matter (up to about 2 percent by weight). The isotopic compositions of the hydrogen and nitrogen atoms in the organic matter are often very unusual. These compositions are best explained if at least some of the organic matter was produced in the interstellar molecular cloud from which the solar system formed.

Other materials that predate the solar system survive in the matrix, albeit at much lower concentrations. Unlike the organic matter, these materials formed not in the interstellar medium but around stars that died millions to hundreds of millions of years before the solar system formed. The evidence that these tiny grains (a few nanometres to 10 micrometres [0.0004 inch] in size) have circumstellar origins lies in their isotopic compositions. These are so different from the compositions of solar system materials that they could only have been produced by nucleosynthesis (formation of elements) in stars. For instance, the average ratio of car-bon-12 to carbon-13 observed in solar system objects is about 89 to 1, with a range of about 85-94 to 1. For some material isolated from chondrites, the carbon-12/carbon-13 ratios of individual particles range from about 2 to 1 to about 7,000 to 1. Types of minerals of circumstellar origin that have been isolated from chondrites include diamond, graphite, silicon carbide, silicon nitride, olivine, corundum, spinel, chromite, and hibonite.

Alteration Processes

Few if any chondrites have remained completely unaltered since they formed as part of their larger parent asteroids. Three processes have modified the chondrites to varying degrees: aqueous alteration, thermal metamorphism, and shock.

Soon after the chondritic parent bodies were formed, they were all heated to some degree. In some bodies, temperatures were modest but high enough for liquid water to exist; reaction of the original minerals with water—aqueous alteration—transformed them to complex mixtures of minerals.

Other chondritic parents were heated more intensely, and, if they once contained water, it was driven off. The temperatures achieved were high enough to induce changes in mineralogy and physical structure—thermal metamor-phism—but insufficient to cause widespread melting. At an early stage, this heating resulted in an increasing uniformity of mineral composition and recrystallization of the matrix. Organic matter and circumstellar grains in the matrix were also destroyed at this stage. With more intense heating, even the chondrules recrystallized. In the most-metamorphosed chondrites examined, those whose parent bodies experienced temperatures of roughly 1,000 °C (about 1,800 °F or 1,270 K), the chondrules are quite difficult to see.

The third modification process, shock, is caused by collisions of meteor-itic parent bodies. Not just chondrites but all major types of meteorites exhibit shock features, which range from minor fracturing to localized melting. The processes of aqueous alteration and thermal metamorphism were probably finished within about 50 million years of the formation of the solar system. On the other hand, collisions of asteroids and their fragments continue to this day.

Classification Systems

The features seen in chondrites reflect processes from two distinct episodes— those that led to the formation of the chondritic parent bodies and those that later altered the material in the parent bodies. As a result, chondrites are classified in two complementary ways. Based on the concentrations of their major elements (iron, magnesium, silicon, calcium, and aluminum) and on their oxidation states, oxygen isotopic compositions, and petrology (e.g., abundance of chondrules and matrix, chondrule size, and mineralogy), chondrites naturally cluster. It is generally believed that the defining characteristics of the classes and groups were determined by conditions prior to and during the formation of the meteorites' parent bodies and that each group comes from a different parent asteroid or set of asteroids.

In addition, within each of the groups, the meteorites differ in the degree to which they were thermally metamorphosed or aqueously altered. These differences are referred to as petrologic types. Types 2 and 1 represent increasing degrees of alteration by water, and types 3 through 6 (some researchers extend the types to 7) reflect increasing degrees of modification by heating. Thus, a meteorite that experienced extensive aqueous alteration would be classified type 1, and one that experienced temperatures just short of melting would be type 6 (or 7). A meteorite that remained completely unmodified by either process since its formation would lie at the boundary of types 2 and 3.

As an example of how the two classification methods are applied, the carbonaceous chondrite known as the Allende meteorite, whose fall was witnessed in 1969, is classified CV3. This indicates that it belongs to the CV group and petrologic type 3.

Meteorites are also classified according to the severity of shock and the terrestrial weathering they have experienced, but these schemes are less commonly used. Still another way to distinguish meteorites is as "falls" or "finds," depending on whether or not they were observed to fall to Earth.

CI Carbonaceous Chondrites

Perhaps the most interesting type of chondrite is the CI group of carbonaceous chondrites. Strictly speaking, it could be questioned why such meteorites are called chondrites at all, inasmuch as they do not contain chondrules. They are aqueously altered so heavily that, if they once contained chondrules, all evidence of them has been erased. When their elemental abundances are compared with those of the Sun, however, it turns out that the two are extremely similar. In fact, of all meteorite types, the CI chondrites most closely resemble the Sun in composition. Consequently, in devising a classification scheme, it makes sense to group them with the chondrites.

Because CI chondrites are chemically so Sun-like—and thus so like the average composition of the forming solar system— some scientists have speculated that they are of cometary rather than of asteroidal origin. Comets are believed to represent the most unaltered material in the solar system. Although there are difficulties with this idea, scientific knowledge about the nature and origin of comets is still limited, which makes it unwise to entirely dismiss this intriguing possibility.

Achondrites

Achondrites, their name meaning "without chondrites," are a relatively small but diverse group of meteorites. They exhibit a range of features that would be expected if their parent bodies experienced widespread melting: igneous features similar to those observed in terrestrial volcanic rocks, segregation of molten metal (possibly into a core) from molten silicate rock (magma), and magmatic segregation of silicate crystals and melt. Most achondrites collected on Earth are derived from asteroids, but one small group is thought to come from Mars and another from the Moon.

The three most numerous asteroidal achondrite groups are the aubrites, the howardite-eucrite-diogenite association, and the ureilites. Aubrites are also known as enstatite achondrites. Like the ensta-tite class of chondrites, the aubrites derive from parent bodies that formed under highly chemically reducing conditions. As a result, they contain elements in the form of less-common compounds—for example, calcium as the sulfide mineral oldhamite (CaS) rather than in its more usual silicate and carbonate forms.

The howardite, eucrite, and diogen-ite (HED) meteorites all seem to be related to one another and probably came from the same asteroidal body, tentatively identified as Vesta, the second largest member of the asteroid belt. They have also been linked to the mesosider-ites, a group of stony iron meteorites. The HED parent body seems to have had a complex history that included melting, segregation of metal into a core, crystallization, metamorphism, and impact brecciation (the process in which an impact shatters rock).

The eucrites are subdivided into cumulate eucrites and basaltic eucrites. Cumulate eucrites are like terrestrial gab-bros in that they seem to have formed at depth in their parent body and crystallized quite slowly. By contrast, basaltic eucrites are similar to terrestrial basalts, apparently having formed at or near the surface of their parent body and cooled relatively fast. The diogenites, composed predominantly of the mineral pyroxene, also seem to have formed at depth. The howardites are impact breccias composed of cemented fragments of diogenite and eucrite materials.

The third main class of asteroid-derived achondrites, the ureilites, are carbon-bearing. They consist of a silicate rock, made primarily of the minerals oliv-ine and pyroxene, that has dark veins running through it. The veins, which constitute as much as 10 percent of the meteorites, are composed of carbon (graphite and some diamond), nickel-iron metal, and sulfides. The silicates clearly crystallized from magma, but there is debate about how they formed. The carbon-rich veins seem to have formed by shock-induced redistribution of graphite that originally crystallized along with the silicates. In addition to the three main achondrite classes, there exist several minor classes and a collection of unique achondrite specimens, all of which reflect the variability of melting processes in the asteroids.

More than 50 meteorites have been identified as having come from Mars, and all are volcanic rocks. All but one of these belong to one of three classes—shergottites, nakhlites, and chassignites—which were established well before a Martian origin was suspected. The three groups are often referred to collectively as SNCs. One piece of evidence for a planetary origin of the SNCs is their young age, between 150 million and 1.3 billion years. To retain enough heat so that volcanic activity could continue until just 1.3 billion years ago, let alone more recently, required a planet-sized parent body. Because there is considerable geochemical evidence that the rocks did not originate on Earth, the only likely candidates that remain are Venus and Mars, both of which appear to have experienced recent volcanic activity.

The most convincing evidence for a Martian origin comes from an Antarctic meteorite, an SNC named EETA79001. This meteorite contains trapped gases (noble gases, nitrogen, and carbon dioxide) whose relative abundances and isotopic compositions are almost identical to those of the Martian atmosphere as measured by the two Viking landers. Scientists believe that the Martian meteorites are fragments of the planet's near surface that were launched into space by large impacts and that eventually found their way to Earth. In the case of EETA79001, atmospheric gases apparently became trapped in glasses produced during the violent shock event that excavated the rock from Mars. As the only samples of Mars available to scientists on Earth, Martian meteorites provide a unique window into the evolution of this enigmatic planet.

Several Martian meteorites have been aqueously altered to some degree, which is in line with other evidence that liquid water was present at least periodically on Mars at some time in the past. The most unique Martian meteorite is another Antarctic specimen, ALH84001. This rock, an orthopyroxenite, has a crystallization age of about 4.5 billion years, which is roughly the same age as asteroidal meteorites, but several of its properties clearly tie it to the other Martian meteorites. About 3.9 billion years ago, aqueous fluids passed through it, precipitating carbonate-magnetite-sulfide mineral assemblages. Some researchers interpreted these rather unusual assemblages as evidence for life on Mars. They also reported features in the meteorite that they interpreted as fossilized bacteria. These claims created considerable controversy, but they also generated important debate on how life might originate and how it might be recognized even if it is unlike the life known on Earth.

A number of lunar meteorites have been found in Antarctica and hot deserts on Earth. They probably would not have been recognized as having come from the Moon were it not for the lunar samples brought back by the manned Apollo and robotic Luna missions. The meteorites, which likely are fragments blasted off the Moon by large impacts, resemble the various rock types represented in the lunar samples (e.g., mare basalts, highland regolith breccias, and highland impact-melt breccias), but they almost certainly came from areas that were not sampled by the various missions. Therefore, like the Martian meteorites, they are an important source of new information on the formation and evolution of their parent body.

Iron Meteorites

Iron meteorites are pieces of denser metal that segregated from the less-dense silicates when their parent bodies were at least partially melted. They most probably came from the cores of their parent asteroids, although some researchers have suggested that metal, rather than forming a single repository, may have pooled more locally, producing a structure resembling raisin bread, with metal chunks as the "raisins." The latter would have been likely to occur if the asteroid underwent localized shock melting rather than melting of the entire body.

Iron meteorites are principally composed of two nickel-iron minerals, nickel-poor kamacite and nickel-rich taenite. The abundances of these two minerals strongly influence the structure of iron meteorites. At one extreme is the class known as hexahedrites, which are composed almost entirely of kamac-ite. Being nearly of a single mineral, hexahedrites are essentially structureless except for shock features. At the other extreme is the class known as atax-ites, which are made up primarily of taenite. Ataxites are the rarest class and can contain up to about 60 percent nickel by weight. Again, because they are nearly monomineralic, they are almost featureless structurally. Between these two classes are the octahedrites. In these meteorites, kamacite crystals form as interlocking plates in an octahedral arrangement, with taenite filling the interstices. This interlocking arrangement, called the Widmanstatten pattern, is revealed when a cut and polished surface of the meteorite is etched with dilute acid. The pattern is an indication that octahedrites formed at relatively low pressure, as would be expected if they formed in asteroid-sized bodies.

At one time iron meteorites were classified in terms of nickel content and Widmanstatten structure, but this has been largely superseded by a chemical classification based on gallium, germanium, and nickel content. The most-common classes have the rather uninspiring names IAB, IIAB, IIIAB, IVA, and IVB. There are numerous other smaller classes and unique iron meteorites. On the assumption that most iron meteorites formed in the cores of their parent asteroids, variations in the composition and properties of iron meteorites in a given class reflect the changing conditions during solidification of these cores. Gallium and germanium abundances in molten nickel-iron metal are relatively unaffected by the process of crystallization, but they are sensitive to the conditions under which the parent asteroid formed. Thus, iron meteorites with similar gallium and germanium abundances are probably related to one another, either because they came from the same asteroid or because their parent asteroids formed at similar times and places. Nickel abundances, on the other hand, are influenced by crystallization because nickel tends to concentrate in those portions of nickel-iron metal that are still molten. As a result, nickel abundances can be used to determine the sequence of crystallization within iron meteorite classes.

The IAB, IIICD, and IIE iron meteorites exhibit geochemical characteristics that are distinct from those of the other classes of irons. Their origin remains uncertain, but they may have been produced by impact processes.

Stony Iron Meteorites

Stony iron meteorites contain roughly equal amounts of silicate minerals and nickel-iron metal. They fall into two groups: pallasites and mesosiderites. Pallasites are composed of a network of nickel-iron metal in which are set crystals of the silicate mineral olivine. Olivine crystals are typically about 0.5 cm (0.2 in) across. The centres of large areas of metal exhibit the Widmanstatten structure. Pallasites formed at the interface between regions of molten nickel-iron metal and molten silicates. The molten nickel-iron metal regions could have been the outer cores of asteroids or, less likely, nuggets in the asteroids where the metal had collected.

Similarly, the molten silicate regions could have been the deepest layers of the silicate mantle. Mesosiderites are probably related to the three classes of achondrites collectively called HEDs. Like one of the HED classes, howardites, meso-siderites are impact breccias containing fragments belonging to the other two classes, eucrites and diogenites. In addition, however, the mesosiderites contain a large amount of dispersed nickel-iron metal. The origin of the metal is not known for certain, but it may be from the core of the body that collided with and brecciated the mesosiderite parent body.

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