Meteorites definition and classification

Definition and classification

Definition

A meteorite is a rocky body of extraterrestrial origin that has survived the passage of the atmosphere and is therefore found on the ground. It is believed that a huge meteorite (several kilometers in diameter) was responsible for the disappearance of the dinosaurs, 65 million years ago. It is estimated that 10,000 tons of micrometeorites and meteorites fall to earth each year.

There is a great variety of meteorites but we can distinguish three main categories :

the stony ones (92% of meteorites),

the irons (6% of meteorites),

the siderolites (2% of meteorites).

The stony ones :

The ordinary chondrites

Chondrites owe their name to the presence of chondrules, small spherules from 0.1 to 10 mm in diameter that are not found in terrestrial rocks, and mainly composed of silicate minerals such as olivine and pyroxene. The whole is joined together in a finely crystallized matrix containing a little iron, sometimes whitish refractory inclusions rich in calcium and aluminium, and in the case of carbonaceous chondrites, a strong proportion of water and carbon.

Chondrites are classified according to their metal content and the degree of fusion of the chondrules between them.

Type and Features :

Type 3 Chondrules all separated (rare)

Type 4 Slight melting

Type 5 Important fusion

Type 6 Complete and almost total fusion

Type 7 Crystalline texture (very rare)

Enstatite chondrites

Type of chondrite rich in enstatite with a metal content of 25 to 35 %.

Classification and characteristics :

Chondrite EH : Presence of mini-chondrules, iron content up to 35 %.

Chondrite EL : Medium-sized chondrules, iron content lower than 12 %.

Carbonaceous chondrites

Carbonaceous chondrites are chondrites rich in carbon. They are classified into several groups according to their concentration in carbon and oxygen, and by reference to typical meteorites. The concentration of oxidized iron and the melting of chondrules are also used to specify this classification.

Classification	Characteristics	Reference drops
Chondrite CB		Bencubbin
Chondrite CH	Presence of micro-chonders, rich in metal, poor in volatiles, mixture of pure iron and carbon (very rare)
Chondrite CI	No chondrules, 3-5% carbon, 20% water, hydrated silicates, magnetite, sulfides, amino acids, organic compounds, density 2.5-2.9	Ivuna meteorite fallen on December 16, 1938 in Tanzania
Chondrite CK	Presence of large chondrules, dark silicates, no metal, much oxygen (rare)	Karoonda meteorite fallen in 1930 in Australia
Chondrite CM	Presence of mini-chondrules, 0.6 to 2.9 % carbon, 13 % water, olivine and pyroxene debris, density 3.4 to 3.8	Mighei meteorite fallen on June 18, 1889 in Ukraine
Chondrite CO	Mini-chonders, 0.21 to 1% carbon, less than 1% water, density 3.4 to 3.8	Meteorite of Ornans fallen on July 11, 1868 in France
Chondrite CR	Agglomerate of primitive chondrules bound by pure carbon, presence of water (rare)	Météorite of Renazzo fallen on 1824 in Italy
Chondrite CV	Présence de gros chondres, l'une de celles qui contiennent le plus d'éléments pré-solaires	Meteorite of Vigarano fallen on january 22, 1910 in Italy
Chondrite C ungrouped

Other chondrites

Other types of chondrites exist, in particular :

Rumuruti (or simply R-type) chondrites, highly oxidized, rich in 17O. The Rumuruti meteorite fell in Kenya in 1934;

Kakangari type chondrites (or simply K type). The Kakangari meteorite fell in India in 1890.

Achondrites

The achondrites are, them, devoid of chondrules. They come from the crust or the mantle of a large asteroid, certainly the asteroid Vesta 4, and they have probably undergone a crystallization from a magma. Among them, we have the SNC meteorites (Shergottites, Nakhlites, Chassigny) and the lunar meteorites.

Classification	Caractéristics
Angrites	Rich in calcium (more than 5 %), rich in titaniferous calcic pyroxene (90 % augite), also composed of troilite and olivine (rare)
Aubrites	Calcium-free, consisting of silica and magnesia, rich in enstatite, density 3.2. Probably a chondrite E melted following a metamorphism (rather rare)
Diogénites	Low in calcium (less than 3%), hypersthene meteorite, pyroxene containing iron and various minerals, density of 3.3 to 3.4
Eucrites	Rich in calcium (more than 5%), one of the most common, similar to terrestrial basalts, rich in pigeonite and calcium feldspar, also composed of troilite, olivine, chromite and ferro-nickel
Howardites	Rich in calcium (more than 5 %), formed by a polymict breccia containing various rocky debris, density of 3.2 to 3.3 (rather rare)
Uréilites	Low in calcium (less than 3%), olivine and pigeonite meteorite containing ferro-nickel, clinopyroxene and sometimes diamond, density 3.3 (very rare)

The primitive achondrites

Classification	Caractéristiques	Chute de référence
Acapulcoïtes	Medium granularity, chondritic abundance of plagioclase and troilite	Acapulco meteorite fallen in Mexico in 1976
Brachinites		Brachina meteorite fallen in Australia in 1974
Lodranites	Coarse grained, subchondritic abundance of plagioclase and troilite
Winonaïtes	Related to silicates	Winona meteorite fallen in Arizona in 1928

Lunar meteorites

A lunar meteorite is an achondrite-type meteorite originating from the Moon. In other words, a lunar meteorite is a rock found on Earth but ejected from the Moon following the impact of a celestial object. We are sure of its origin by comparison with the samples brought back from the Moon by NASA.

The Martians

Classification	Caractéristics
Chassignites	Rich in calcium, composed mainly of olivine, plus some oxidized elements and hydrated minerals. Meteorite supposed to come from Mars
Nakhlites (en)	Rich in calcium, composed essentially of augite, plus some oxidized elements and hydrated minerals. Meteorite supposed to come from Mars
Shergottite	Rich in calcium, basaltic rock composed mainly of pyroxene and plagioclase, plus some oxidized elements and hydrated minerals. Meteorite supposed to come from Mars

Iron meteorites:

Iron meteorites, sometimes called ferrous meteorites or siderites, are a type of meteorite composed primarily of a metallic alloy of iron (Fe) and nickel (Ni). They are interpreted as fragments of asteroid cores that have been literally peeled from their silicate mantle by collisions with other solar system objects.

According to their chemical composition, they are grouped into three types: octaedrites, hexaedrites and ataxites:

- octaedrites: which present, after sawing, polishing and acid attack, Widmanstätten figures, are the most numerous siderites;
- hexaedrites : less rich in nickel (5 to 6 %), only rarely show striations, sometimes Neumann lines;
- ataxites: rich in nickel (16% minimum), whose Widmanstätten figures are invisible to the naked eye.

Siderolites:

A siderolite (also called lithosiderite, ferro-stone meteorite or mixed meteorite) is a meteorite composed for half of an iron-nickel alloy and for half of silicates. It is in fact a stony meteorite with metallic incrustations.

The mixed meteorites are divided into two subclasses:

- the pallasites ;
- the mesosiderites.

These meteorites come from the zone between the metallic core and the rocky mantle of a large asteroid. They are formed by the sedimentation of dense mineral elements (olivine) in molten metal. Slowly, the whole cools and solidifies.

Classification	Metal content
Chondrite H	15 to 25 %, density 3.4 to 3.6, bronzite chondrite and olivine
Chondrite L	7 to 15 %, density from 3,6 to 3,9, chondrite with hypersthene and olivine
Chondrite LL (Amphotérites)	3 to 7 %