Chondrite Ci — Wikipedia

THE Chondrites CI or CI chondrites , sometimes called chondrites C1 , are stony meteorites belonging to carbon chondrites. Of all meteorites, their chemical composition shows the greatest affinity with the elementary abundance of the sun (photosphere).

The abbreviation Ci is derived from C from English carbonaceous and I d’ Hear , the typical locality in Tanzania. THE first in C1 indicates type 1 in the classification of Van Schmus-Wood.

There are very few chondrites here, in 2021 we only know nine ^{[ first ]} :

• The meteorite of Alais, fell in 1806 near Alès (formerly Alais) in France, from which fragments were recovered in the neighboring municipalities of Saint-Étienne-de-l’Olm and Castelnau-Valence, weighing together 6 kg ;
• The pride meteorite, which fell in 1864 in Pride, near Montauban. She disintegrated in twenty songs weighing 14 kg in total ;
• Tonk’s meteorite, which fell in 1911 near Tonk in Rajasthan, India. Only a few fragments were collected in total more than 7.7 g ;
• The meteorite of Ivuna, the lithotype, fallen in 1938 near Ivuna, in Tanzania, fragmented into three pieces weighing 705 g in total ;
• The Revelstoke meteorite, which fell in 1965 near Revstoke in British Columbia (Canada). His fall was very brilliant but there were only two small pieces of 1 g ;
• The meteorites Yamato 86029, Yamato 86737, Yamato 980115 and Yamato 980134, found in the Yamato mountains, in Antarctica.

By adding all these events, there is less 17 kg ^{[Ref. necessary]} of chondritic material Ci.

During the Apollo 12 mission in 1969, a meteorite was discovered on the moon. At first we thought of chondritis of the type, but later it was established as a CM chondritis. In 2000, a well -observable fall came at Lake Tagish in Yukon. This meteorite is now one of the CI chondrites, but it contains (unlike others) chondres. It was therefore designated Ci2.

The CI chondrites are very brittle and porous rocks, which easily disintegrate during their crossing of the earth’s atmosphere. Consequently, there were mainly only small fragments, a good example being the fall of Revestoke which was very spectacular but with a tiny harvest (not even a gram!). CI chondrites are characterized by a black fusion crust which is very difficult to distinguish from the matrix of the same color. The matrix is ​​opaque and very rich in carbon. In addition, it contains black minerals such as magnetite and pyrrhotitis. In places, white minerals rich in water (such as carbonates and hydrated sulfates) are incorporated.

CI chondrites are most characterized by the absence of recognizable chonders. Small chonding fragments and mineral inclusions rich in calcium and aluminum (in English calcium-aluminium rich inclusions – CAI) are undoubtedly rare but they exist.

The mineralogy of CI chondrites is dominated by a fine matrix, rich in phyllosilicates housing carbonates, sulfates, sulfides and magnetite. CI chondrites show the following mineralogy.

Mainly:

Ferromagnesians:

All ferromagnesian minerals are small, equidomentional and autumorphic grains, crystallized at high temperatures.

Oxides:

• magnetite. Developed in raspberry, spheures and plates. Very abundant.

Sulfides:

Water alteration minerals:

Carbonates of calcium, magnésium and out.

Magnesium and calcium hydrated sulfates.

Carbon minerals. They contain inorganic and organic substances:

Ferromagnesian minerals are isolated and have no alteration ^{[ 2 ]} . On the other hand, for the genesis of phyllosilicates and substances resembling serpentine, we admit the water alteration acting on olivine and pyroxenes rich in magnesium ^{[ 3 ]} .

Ci chondrites are very rich in water, they contain between 17% and 22% (weight). Their great porosity (up to 30%) is probably correlated with this. Water does not appear in free condition but is part of the structure of hydrated silicates. A very strong alteration at low temperatures (between 50 and 150 ° C) ^{[ 4 ]} – A characteristic of CI chondrites is indicated by the appearance of minerals such as Epsomite or by carbonates and by sulfates. Certainly, water crossed the parental body through existing slots and cracks.

Iron is present with 25% (weight), but especially in oxidized condition (magnetite). Some iron sulfides (pyrrhotitis, cubanite, pentlandite and troilite) are present, but the elementary iron is absent ^{[ 5 ]} . The MG/if 1.07 relationship is quite high ^{[ 6 ]} . Only CV chondrites are even more enriched with magnesium. With a value of 0.057, the CI chondrites have the lowest relationship Ca/if all carbon chondrites ^{[ 7 ]} . Isotopes of oxygen (Δ ¹⁷ O and Δ ¹⁸ O) show the highest values ​​of all carbon chondrites. At the same time, the relationship δ ¹⁷ O/d ¹⁸ O is comparable to terrestrial values.

CI chondrites also contain 3.5% (weight) carbon. Carbon can be of inorganic (graphite, diamond and carbonates) or organic (amino acids, hydrocarbons and porphyrines). Organic components are of prime importance for the origin of life on earth.

Because of their high porosity, CI chondrites only have a density of 2.2 g/cm ³ .

Among all meteorites, CI chondrites show the greatest similarity to the elementary abundance of the solar nebula. For this reason, they are designated Primitive meteorites . Volatile elements such as carbon, hydrogen, oxygen, nitrogen and noble gases are depleted in the CI chondrites; All other elements have practically the same abundance. Another exception is lithium, which is still enriched in meteorites (during nucleosynthesis, the sun uses lithium to produce heavier elements).

Because of this great similarity, we normalize the samples in petrology towards the CI chondrites. The sample/chondrite relationship is used to compare the abundance of an element with abundance in the solar nebula. The values> 1 indicate an enrichment, the values ​​<1 A impoverishment towards the original matter. This method is applied especially in the spider diagrams for rare earths.

CI chondrites and CM chondrites, close to composition, are very rich in volatile material, especially in water. It is assumed that they were formed in the outer part of the asteroid belt, at a distance surpassing 4 AU. At this distance is the ice line with an ambient temperature of 160 K. Any trace of condensa liquid water at these conditions and is preparing by becoming ice. Thus explains the great similarity of the chondrites here with the frozen moons of the external solar system. In addition, it seems that there is a connection between the CI chondrites and the comets; Like comets, CI chondrites accreated silicates, ice, other volatile substances and organic components (see: Halley comet).