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Information about meteorites

Meteorite

A meteorite is a rock that originates in outer space before reaching the ground. Whilst this rock is still in space it is called a meteoroid. When it enters the Earth’s atmosphere it becomes a meteor. The fireball it forms as a meteor is colloquially referred to as a shooting star. Meteorites give us detailed information about the composition, history and appearance of other celestial bodies. This is crucial to understand the origin of the Earth, Solar System and entire universe. Meteorites also give us information about how physical and chemical reactions happen in space, some of which are necessary for the development of life.

Meteorites, specially when they are fresh, have peculiar characteristics such as fusion crust. These features are indicators of the last dramatic seconds of a meteorite’s journey into the Earth, when crossing the atmosphere. You can read more about meteorites’ flight features.

Currently, scientists are dividing meteorites into 2 groups: undifferentiated and differentiated: whether or not major chemical or physical changes in large parent bodies occurred. Chondrites (see below) comprise the entire undifferentiated group and the rest of meteorites the differentiated group. This division is necessary for scientists as it is precise. There is a more visually intuitive classification that was once used in science and many people still follow that is based on the appearance. In this classification there are 3 main types of meteorites: stony, iron and stony-iron.

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Easy meteorite clasification
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Extended meteorite classification
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Complex meteorite classification

1. Stony

Stony meteorites consist predominately of silicon-based minerals that form rock. They are the most abundant type of meteorite with about 95% of meteorite falls belonging to this group. They are divided into two subgroups: chondrites and achondrites.

1.1. Chondrite

Chondrites are so-called because they contain chondrules. Chondrules were created when molten droplets of silicates solidified. Then they accreted (form by gradual accumulation or coalescence) in space together with free metals to form the first asteroids 4550 million years ago. Chondrites account for approximately 93% of stony meteorites. They are older than the Earth and the most abundant and complex type of stony meteorite.

Chondrites have different degrees of metamorphism and are thus divided into different petrologic types. They are categorized numerically from 1 to 7 in which 3 is the virtual center. Some scientists only consider numbers from 1 to 6.

Aqueous metamorphism is represented by the numbers 1 and 2, with 1 having obliterated chondrules and 2 containing altered chondrules.

Thermal metamorphism is represented by the numbers 3 to 7, with 3 being unaltered pristine chondrules and 7 being destroyed chondrules. Numbers 4, 5 and 6 are graduations between the two.

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Stony meteorite

1.1.1. Ordinary

Ordinary chondrites are the most common type of chondrites. This type represents about 94% of all chondrites, 87% of stony meteorites in general, and 83% of all meteorites. They are divided according to the amount of iron within their structures and by the degree of thermal metamorphism (read chondrite). Depending on their metallic iron content, they are divided into 3 groups: H, L and LL. H means high iron content, L is low iron content and LL is very low iron content. Consequently, an ordinary chondrite with very well defined chondrules and a high metallic iron content would be classified as an H3. On the other hand, if it has no apparent chondrules and a very low iron content it would be an LL6 or LL7. If the chondrite has semi-defined chondrules and a low iron content it would be classified as an L4 or L5.

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L6 ordinary chondrite slice

1.1.2. Carbonaceous

This is a complex group of chondrites which has many subgroups. The meteorites in this group were originally named carbonaceous because of their supposed higher carbon content. Later this was found to not be true in the case of all subgroups. The name ‘carbonaceous’ was maintained, but their scientific differentiation from ordinary chondrites is now based their higher abundance of refractory elements like calcium and aluminum. Carbonaceous chondrites come from very diverse locations in our Solar System and they can exhibit both thermal and aqueous metamorphism (read chondrite). Each type is abbreviated with a C for ‘carbonaceous’ then another letter indicating its subgroup. This second letter derives from the fall location of the first meteorite described for each subgroup (known as type-specimens). Therefore, meteorites that are like the carbonaceous meteorite that fell in Vigarano (Italy) are named CV. The ones like the Mighei (Ukraine) carbonaceous meteorite are CM. Other subgroups are Karoonda (CK), Ivuna (CI), Renazzo (CR), Ornans (CO), and Bencubbin (CB). There is only one exception to this nomenclature rule and that is a rare carbonaceous chondrite which is high in iron and is abbreviated CH. Carbonaceous chondrites are abundant in fascinating ingredients. Some have a high water content, amino-acids and sugars. They also have amazing materials and inclusions. The Solar System is 4800 million years old – some presolar grains can be 7000 million years old. Carbonaceous meteorites are presumably the scientifically more important type of meteorite.

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CV3 meteorite slice

1.1.3. Enstatite

Also known as E chondrites, enstatite chondrites account for about 2% of all chondrites. They take their name from the mineral enstatite, of which they have a high content. They differ from ordinary chondrites primarily by having almost no iron oxide content. Interestingly, enstatite chondrites are the driest objects in the Solar System, consisting of only 0.01% water. Asteroid 16 Psyche has been proposed as the parent body of E chondrites. Their petrologic types range from 3 to 7 and they have two main subgroups: high and low iron which are abbreviated as EH and EL respectively.

1.1.4. Rumuruti

Also known as R chondrites, they are a very rare type of chondrite accounting for only 0.4% of all chondrites. They are abbreviated with an R, after the Rumuruti meteorite which fell on 28th January 1934 in the Rift Valley, Kenya. It consisted of a single piece weighing only 67 grams. R chondrites differ from ordinary chondrites mainly because most of their metal content is in the form of sulfides, they are more oxidized, and contain very little metallic iron and nickel. They have a matrix with more dust (about 50%), a higher trace element concentration of zinc and selenium, and a higher oxygen-17 ratio. Implanted solar wind is found in more than half of the rumuruti chondrites that have been analyzed for noble gases. This suggests that R chondrites come from the regolith of an asteroid (a surface with loose solid materials).

1.1.5. Kakangari

Also known as K chondrites, they are an exceptionally rare type of chondrite accounting for less than 0.01% of all chondrites. They are abbreviated with a K, after the Kakangari meteorite which fell on 4th June 1890 in India. It left just two pieces weighing only 350 grams. Kakangari chondrites differ from ordinary chondrites primarily by their reduced silicate mineralogy.

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Easy chondrite meteorite classification
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Extended chondrite meteorite classification

1.2. Achondrite

Achondrites are stony meteorites that do not contain chondrules. Their material has been differentiated due to melting and recrystallization in large parent bodies. They account for about 6.5% of stony and 6% of all meteorites. It is a complex group that contains meteorites from diverse locations throughout our Solar System. There are divided in lunar, martian, asteroidal and primitive.

1.2.1. Lunar

Lunar meteorites result from another meteorite hitting the Moon, causing it to release its rocks at escape velocity into space. They go into orbit for probably millions of years until their trajectory led them to land on Earth. These meteorites are known to be from the Moon because lunar rocks were collected and brought to Earth by the Apollo missions. Their composition was tested, and it was scientifically proven that they cannot have originated in any part of the Solar System other than our Moon. Although the Moon is our nearest celestial body, lunar meteorites are extremely rare due to the chain of events necessary for them to find their way to Earth. Only 0.6% of all known meteorites are lunar and there has never been an observed fall of a lunar meteorite. Now, however, you can have a piece of the Moon in your hands.

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Lunar meteorite slice

1.2.2. Martian

Martian meteorites result from another meteorite hitting Mars, causing it to release its rocks at escape velocity into space. They go into orbit for probably millions of years until their trajectory led them to land on Earth. These meteorites are known to be from Mars because they have tiny bubbles of air that are unequivocally similar to the Martian atmosphere as studied in-situ by Viking landers. There are 3 main types of Martian meteorites: Shergottites, Nakhlites and Chassignites that form the SNC group. There are others types that are much rarer with only 1 meteorite. Although Mars is our third nearest celestial body, Martian meteorites are extremely rare due to the chain of events necessary for them to find their way to Earth. Only 0.4% of all known meteorites are Martian. Now, you can have a piece of the Mars in your hands.

1.2.3. Asteroidal

This group comprises achondrites that come from large differentiated parent bodies where their mineralogical and chemical composition were changed by melting and crystallization processes. There are 3 groups: HED, angrites and aubrites.

1.2.3.1. HED

They come from Asteroid Vesta. A great impact occurred in the South hemisphere of Vesta less than 1 billion years ago, ejecting rocks from the asteroid into space. Eventually some of these rocks fell on Earth. HED is an acronym for the meteorites that comprise this clan: Howardites, Eucrites, and Diogenites. Eucrites come from the outermost crust of the asteroid, diogenites originated below the crust and howardites are essentially a mixture of the other two types, where more than 10% of the least abundant type is present. Only 3% of all known meteorites belong to the HED clan.

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Provenance of Asteroid Vesta meteorites
1.2.3.2. Angrite

They are basaltic rocks composed mostly from the mineral augite. They can be porous, which is a rare feature among meteorites. Angrites are named after the Angra dos Reis meteorite, which fell in Brazil in 1869.

1.2.3.3. Aubrite

Aubrites are sometimes referred to as enstatite achondrites due to their almost monomineralic enstatite pyroxenite content. They have an igneous origin, and this means they originated in an asteroid. Aubrites are named after the Aubres meteorite, which fell in France in 1836.

1.2.4. Primitive

This group comprises achondrites whose chemical composition is primitive in the sense that it is similar to the composition of chondrites, but their texture is igneous, indicative of melting processes. There are 4 groups: brachinites, the acapulcoite-lodranite family, ureilites and winonaites.

1.2.4.1. Brachinite

Brachinites are a group of meteorites that are classified either as primitive achondrites or as asteroidal achondrites. Like all primitive achondrites, they have similarities and differences with both chondrites and achondrites. Brachinites contain between 74% and 98% olivine by volume. Brachinites are named after the Brachina meteorite, which was found in Australia in 1974.

1.2.4.2. Acapulcoite-Lodranite family

These are closely related to each other and equigranular meteorites. Acapulcoites have a finer grain than lodranites. Acapulcoites are named after the meteorite that fell in Acapulco, Mexico in 1913. Lodranites are named after the meteorite that fell in Lodran, Pakistan in 1868.

1.2.4.3. Ureilite

Ureilites are composed mostly from olivine and pyroxene. However, they are especially noteworthy due to their composition of around 3% carbon, in the form of graphite and nanodiamonds. They are named after the Novo-Urei meteorite that fell in Russia in 1886. The Novo-Urei meteorite was broken apart and partly eaten by witnesses of the fall.

1.2.4.4. Winonaite

Winonaites are fine to medium grained, mostly equigranular, and contain ferric sulfide. They are closely related to the silicate inclusions found in IAB and IIICD irons. Winonaites are named after the meteorite that was found in Winona, USA in 1928. This meteorite was located in a native American stone burial cist, so it was likely highly revered. It probably fell between 1070 and 1275.

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Easy achondrite meteorite classification
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Extended achondrite meteorite classification

2. Iron

Iron meteorites come from the core of dead asteroids. Heavy metallic elements sank towards the center of these asteroids due to gravity. Collisions with other bodies then destroyed these asteroids, releasing their metallic cores to space. Iron meteorites are alloys, mainly of iron and nickel, although they also contain trace elements such as iridium. Iron meteorites account for only 4% of all observed meteorite falls, by number. However, due to their high density, the total weight of iron meteorites accounts for almost 50% of observed meteorite falls. This percentage increases up to 83% if the total weight of meteorite finds is included as well. Current scientific classification for iron meteorites is complex and exclusively dependent on the meteorite’s chemical composition. It includes 15 groups as follows: IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA, IVB, and Ungrouped irons. This classification is necessary for scientists as it is precise. There is a more visually intuitive classification that some collectors follow that is based on the internal structure. According to their structures, iron meteorites are classified into 3 types: octahedrites, hexahedrites and ataxites.

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Iron meteorite

2.1. Octahedrites

They display lines (most easily visible after polishing and etching) which are called Widmanstatten pattern. The Widmanstatten pattern reflects light from different angles due to differences in the crystallization of kamacite and taenite minerals. This is the result of the cooling-down of an asteroid’s core at about 1 degree Celsius every million years. The resulting lines of crystallization form an octahedron (a 3-dimensional shape), from which this type’s name is derived. Some types of octahedrites also display Neumann lines although they are more characteristic of hexahedrites.

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Iron octahedrite

2.2. Hexahedrites

They display a feature called Neumann lines. These lines are very fine and result from a strong mechanical shock in space due to an impact. Hexahedrites are lower in nickel than octahedrites, usually around 5.5%. When crystallized, kamacite forms right-angled cubic crystals with six sides.

2.3. Ataxites

They are a type of iron meteorite that do not show lines upon etching. Their name means ‘without structure’. Ataxites have >18% nickel content and most of the meteoric iron is kamacite with minor amounts of taenite, so no structural difference can be seen when they are cut and etched.

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Iron meteorite classification

3. Stony-iron

Stony iron meteorites are thought to have originated at the core-mantle boundary of differentiated asteroids, where the metallic core meets silicated rocks. They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. The proportion of silicates to metal is about 50% for each. However, it is not uncommon to randomly see stony-iron meteorites with a higher content of one than the other.

3.1. Pallasite

Pallasites are considered by many to be the most beautiful type of meteorite. They are thought to have originated at the core-mantle boundary of differentiated asteroids, where the metallic core meets lighter silicated rocks (olivines). They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. There are 3 sub classes. Main group (90% of pallasites), Ungrouped (6%) and Eagle Station group (4%). Pallasites consist of a metallic matrix with a high quantity of olivine inclusions. Due to their aesthetics when cut, they are often used in jewelry. They also make attractive pieces when cut in half or sliced. 

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Pallasite meteorite

3.2. Mesosiderite

Mesosiderites are thought to have originated at the core-mantle boundary of a differentiated asteroid, where the metallic core meets the lighter rocks. They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. Mesosiderites have similar parts of metal to rock. It is a chaotic mixture, some parts being rock with metal inclusions and others being metal with rock inclusions. There are 3 sub classes (A, B and C) which are based on textural and mineralogical differences. Asteroid 16 Psyche is considered the main candidate as the parent body for mesosiderites. It orbits the Sun between Mars and Jupiter in the asteroid belt.

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