Achondrites

Achondrites are a class of stony meteorites containing abundant silicate minerals formed due to igneous processes on small planetoids or asteroids. They closely resemble basaltic rocks found on the Earth and Moon.

Overview

Meteorites are solid materials from outside Earth’s orbit that have passed through the atmosphere and reached the surface. These objects—made of various combinations of rock and metal—are called meteoroids while still in orbit around the Sun. When a meteoroid encounters the Earth and enters the atmosphere, collisions with air molecules cause it to heat up and begin to vaporize. This produces a glowing trail of superheated air and hot vaporized material from the meteoroid that appears as a flash of light streaking across the sky. This phenomenon is properly called a meteor. (It is also commonly called a falling star or shooting star, but such names are inaccurate since it is not a star.) It is estimated that the Earth is bombarded by hundreds of tons of meteoroidal material every day, but most meteoroids are no larger than small pebbles, and they vaporize completely in our atmosphere. If the meteoroid is large enough to survive its fiery plunge and land on the Earth’s surface, it is called a meteorite.

Meteorites are divided into three groups based on the abundance of metallic and stony minerals contain nickel-iron meteorites (often called iron meteorites or irons), stony-iron meteorites (stony-irons), and stony meteorites (or stones). Stony meteorites are composed primarily of silicate and oxide minerals with minor amounts of metal. They can be subdivided into two subgroups known as the chondrites and achondrites.

Achondrites are less common than chondrites; only about one in ten stony meteorites is an achondrite. Achondrites get their name because they lack chondrules, mineral droplets that make up much of the material in chondrites. Achondrites have crystal textures similar to terrestrial igneous rocks, thus indicating that they formed when some larger parent body (perhaps the size of a small planet or large asteroid) melted, differentiated, and then cooled and solidified. Some achondrites have large mineral crystals that result from slow cooling as intrusive rocks inside the parent body. Others with smaller crystals were formed by more rapid cooling on or close to the surface of the parent body. Some resemble terrestrial lava flows riddled with bubble holes called vesicles caused by gases that escape. Some achondrites show evidence of collisions in space, resulting in a rock called an impact breccia that shows the effects of shock metamorphism. The impact pressure causes the rocks to break apart and the minerals to shatter or deform. In contrast, the heat generated causes mineral and rock fragments to melt slightly or fuse, depending upon its intensity.

Most achondrites are rich in one or more silicate minerals, such as olivine (a magnesium-iron silicate), pyroxene (an iron-magnesium-calcium silicate), and plagioclase feldspar (a calcium-sodium-aluminum silicate), in varying proportions. Other minerals, such as spinel and chromite (iron-magnesium-aluminum-chromium oxides), are also found, as are small amounts of metal (less than 10 percent) in iron and nickel alloys. Generally, achondrites resemble a rock called basalt, a very common dark-colored igneous rock found on the Earth and Moon. With a few exceptions, achondrites are older and contain rare isotopes that make them very different from the rocks on the Earth or the Moon.

Achondrites can be subdivided into several types based on their texture and chemical composition. First and most abundant are the eucrites. They are similar in appearance to fine-grained terrestrial basalts and contain roughly equal amounts of calcium-rich silicate minerals, such as plagioclase feldspar and pyroxene. In a hand specimen, a few eucrites exhibit a cumulate texture formed by the accumulation of coarse-grained crystals within a magma chamber. Still others possess a vesicular texture (containing many bubble holes or vesicles formed by escaping gases) and closely resemble terrestrial basaltic lava flows. Many eucrites also contain mixed fragments from other meteorite types and show the effects of shock metamorphism. The small variation in the abundance of major chemical elements within all eucrites suggests their origin in the same parent body. They probably formed as extrusive and shallow intrusive igneous rocks later blasted out of the parent body by impacts. The age of eucrites has been dated using radioactive rubidium-strontium isotope techniques at 4.5-4.6 billion years, indicating crystallization very early in the solar system's history.

Diogenites are achondrites with a chemical composition similar to a terrestrial igneous rock called pyroxenite, which has abundant pyroxene. Texturally, diogenites have coarse-grained crystals that indicate slow, deep cooling below the surface. Based on laboratory melting experiments using actual achondrite samples, these crystals are probably formed by cooling and crystallization from the same magma that also produced eucrites or by the more extensive melting of some eucrite source. Chemically, diogenites consist of metamorphosed accumulations of an iron-and-magnesium-rich but calcium-poor pyroxene known as bronzite, along with minor amounts (less than 10 percent) of plagioclase feldspar crystals and some metallic iron. The bronzite crystals have become chemically homogeneous as a result of metamorphic heating. Like some eucrites, diogenites have been found shattered or mixed with pieces of other meteorites, resulting in a solid rock of angular broken fragments. Both diogenites and eucrites probably formed on the same parent body.

Howardites are a variety of achondrites that represent mixtures of many different meteorite types. They consist of crushed pieces from eucrites and diogenites, and they also contain about 2-3 percent by weight of pieces from chondritic stony meteorites. Texturally, howardites closely resemble the lunar soil. Under high magnification, a howardite’s exterior surface is covered with small micrometeorite craters that contain impact-generated glasses, evidence of their formation by impacts on the surface of the parent body.

A likely parent body for these three types of achondrites seems to be the asteroid 4 Vesta. About 500 kilometers in diameter, it is the only large asteroid with a surface reflection spectrum like eucrites and diogenites. Several small Earth-approaching asteroids have similar reflection spectra, and they and the eucrites, diogenites, and howardites found on Earth probably were blasted off of Vesta by one or more large impacts. Further, research published in October 2024 indicated that approximately 70 percent of Earth's meteorites originated from at least three significant asteroid breakup events.

Ureilites, another variety of achondrite, were named for Novo Urei, Russia, where the first specimen was found in 1886. They consist of fairly large and abundant crystals of magnesium-rich olivine, some clinopyroxene, and a rare type of plagioclase feldspar set within smaller crystals composed of graphite, iron-rich metals, halite, sylvite, and troilite. In some specimens, the olivine crystals show a preferred orientation from crystal settling while molten; thus, ureilites exhibit variable textures. Most specimens have undergone intense high-pressure shock metamorphism that formed small diamonds from the graphite. Ureilites are the only achondrites containing these tiny graphite and diamond crystals; the carbon source is unknown.

Aubrites, also known as the enstatite achondrites, are composed predominantly of magnesium-rich pyroxene called proto enstatite and a rare type of plagioclase feldspar. Texturally, aubrites have large crystals, indicating slow cooling, but their origin and place of formation remain unexplained.

SNCs (pronounced “snicks”) are a small, unusual, and highly controversial group of related achondrites, including the shergottites, nakhlites, and chassignites. Shergottites are named for the town of Shergotty in the state of Bihar in India, where the first of these strange meteorites fell in 1865. Since that time, a few others like it have been found. The shergottites are similar to a terrestrial, slow-cooled, coarse-grained igneous rock called diabase, which is rich in pyroxene and plagioclase feldspar. One of the feldspars found within the shergottites is maskelynite, a type of feldspar whose orderly atomic lattice structure has become disorganized from shock impact. Other minerals to be found are pyroxenes (calcium-rich augite and calcium-poor pigeonite), calcium- and sodium-rich plagioclase feldspars, oxidized iron in the form of magnetite, some olivine, and a rare water-bearing amphibole named kaersutite. Texturally, the shergottites are cumulated with elongated pyroxene crystals that have a preferred orientation, probably due to the flowage of newly formed crystals within the magma while still in a hot liquid state. Their geologic history records crystallization in a relatively Earth-like oxygen-rich environment and a period of intense shock metamorphism and high-intensity heating probably caused by impact, as indicated by numerous quickly cooled glass fragments. The radiometric age determinations on some of the shergottites’ minerals reveal a comparatively young age of 1.3 billion. Trapped gas bubbles within some of the shergottite samples contain nitrogen and noble gases, such as xenon, krypton, and argon, which are very similar in composition to the atmosphere of Mars.

The nakhlites are similar to terrestrial slow-cooled, coarse-grained igneous rocks known as gabbro. Mineralogically, these meteorites contain abundant augite (calcium-rich pyroxene) and smaller amounts of olivine, plagioclase feldspar, a few strange sulfide minerals, and metallic iron. Compared to the shergottites, all these minerals lack shock features, show no evidence of thermal metamorphism but have a similar radiometric age of 1.4 billion years. Although their overall chemistry differs from the shergottites, nakhlites are believed to have been derived from either the same or a similar parent body.

Chassignites are named for Chassigny, France, where the first specimen was found in 1815. Several others have since been found in localities around the world. The few existing samples show that they are composed of abundant crystals of olivine with minor amounts of pyroxene, plagioclase feldspars, and kaersutite that show alteration by shock metamorphism. In a hand specimen, chassignites closely resemble terrestrial olivine-rich rocks called dunite.

The SNCs contain cumulate crystals and a large percentage of volatile gases, indicating formation upon a planet-sized body with a stronger gravitational field than that of the Moon. As a group, they have an average age of about 1.3-1.4 billion years, as determined by radioactive dating techniques. The chemical abundance and isotopic composition of gases trapped within small bubbles in these meteorites are nearly identical to the Martian atmosphere. The SNCs may have formed on Mars, from which they were blasted off into space by large impacts that hit Mars at the correct angle and speed to eject material at speeds greater than five kilometers per second (Martian escape speed); these Martian rocks would eventually have encountered Earth and landed here as meteorites. Specimens from the surface of Mars will first need to be returned to Earth before it is possible to confirm that SNCs originated there.

In 1974, a single strange meteorite named Brachina was found in Australia. It is similar to the chassignites in mineralogy. Brachina has a fine-grained texture, contains 80 percent olivine and 10 percent plagioclase feldspar, lacks hydrous minerals, is unshocked, and is 4.5 billion years old (much older than any of the SNCs).

Methods of Study

Meteorites arrive daily on the Earth in sizes ranging from specks of dust to huge masses of several thousand kilograms. The vast majority fall into the ocean, never to be recovered, or into remote, uninhabited areas to be discovered much later. Most achondrite meteorites probably pass by unnoticed because they closely resemble ordinary Earth rocks.

The best place to find meteorites of all types is Antarctica. In this remote, ice-covered continent, meteorites stand out starkly as black rocks against a white background of snow and ice. Because of the extreme cold and lack of liquid water, nearly all varieties of meteorites are found perfectly preserved. Meteorites are usually named for the closest town or post office in the vicinity where they are found; however, in the case of Antarctica, the name of the nearest mountain range, valley, or other topographic feature is used.

In the laboratory, the meteorite is weighed and measured, its density determined, and its physical appearance described. Thin sections are sliced from small chips of the meteorite for viewing under a petrographic microscope, where the behavior of light passing through the individual crystals of the specimen assists in identifying the minerals. The bulk chemical composition of the meteorite and a detailed analysis of its minerals can be made using an electron microprobe. When bombarded by an electron beam, the atoms within the specimen emit X-rays. The atoms of each element emit X-rays with characteristic energies, and the intensity of each X-ray energy indicates the abundance of the corresponding element. The overall texture of the meteorite and the distribution and abundance of each mineral present in it are used to place it in the classification system. Its bulk chemistry and elemental distribution help determine the processes that created it.

Extensive studies suggest that most achondrites probably came from water-free planetoids or asteroids. Based on meteorite melting experiments in the laboratory, the partial melting of a parent body with an overall chondritic composition could produce a eucrite. Other experiments using partially melted igneous rocks containing olivine and plagioclase feldspar produced magmas that could form diogenites and eucrites under the proper conditions. Melting could easily have taken place in the low-pressure environment of space, provided that enough heat was generated by the decay of short-lived radioactive isotopes, such as aluminum 26, that once were abundant in these rocks.

Applying processes similar to those scientists believe formed the Earth. A parent body may be modeled that extensively melted and became partially separated into an upper layer of eucrite material atop a lower layer of diogenite material, surrounding a small, metallic iron-nickel core. The size of this parent body probably was no larger than a few hundred kilometers, so pressures on the interior core region would not have been more than two to three kilobars (two to three thousand times Earth's atmospheric pressure at sea level). The mixing of eucrites and diogenites to form howardites probably occurred via meteorite impact, excavation, and lithification on or close to the surface of the parent body.

In January 2024, scientists discovered that meteorites that fell near Berlin were aubrites, a rare group of achondrites. Aubrites are known for their unique mineralogy. This discovery provided insights into their parent bodies.

Context

Meteorites are samples of the building blocks of the planets. They have provided evidence for reactions in the solar nebula before the formation of the planets, processes occurring in planetlike bodies during their formation, and collisional impact events between solar-system objects. Continued study of the achondrites and other meteorite types will provide more clues as to how the planets formed about 4.5 billion years ago.

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