Lithoids, non-ferrous meteorites

The lithoids are a family that includes non-ferrous meteorites; it is the largest with nearly 93% of all known specimens. The lithoids include:

  • achondritesachondrites, whose crustcrust is smooth;
  • carbonaceous chondrites, which can be recognized by their blistered crust.

However, it is the microscopic or chemical analysis that is often decisive.

The lithoids do not contain iron or nickel. They are much more fragile than siderites and can disintegrate on contact with the ground. Made up of differentiated or totally reprocessed matter, they are formed by fusion then recrystallization-recrystallization in the core or on the surface of a parent star. This results in a distinct structure and mineralogy typical of an igneous process.

Achondrites mainly contain siliconsilicon and silicates such as magnesiummagnesium oxide. There are also traces of chlorchlorine, potassiumpotassium, scandiumscandium, titaniumtitanium, vanadiumvanadium, manganese, etc. The presence of enstatite gives them a white internal color.

77g Achondrite eucrite Millbillillie, a monomictic breccia results from rock deformation by shearing and granulation (cataclasis) in the process of tectonism or – more general – dislocation metamorphism. If the dislocation metamorphism is impact-related, the produced cataclasite may be termed a monomict impact breccia. Dr. Svend Buhl, Niger Meteorite Recon, CC BY 3.0, via Wikimedia Commons

Chondrites: carbonaceous, ordinary and enstatite

Isotope dating gives the chondrites an age of 4.55 billion years. This is the approximate age of the Solar System. They are therefore the rare virginal representatives of the raw material which fashioned the Solar SystemSolar System (even if, as we shall see, in many cases their native properties have been altered by heat or cold). Their study is therefore particularly interesting for determining the primitive composition of the Solar System.

This type of meteorites is composed of 15 to 75% of chondreschondres (from the Latin chondros which means “grain of sand”), kinds of small clear inclusions composed of olivine and pyroxene in different proportions and structures.

Cut Chondrite meteorite, part of its bark is visible on the surface. Tiia Monto, CC BY-SA 3.0, via Wikimedia Commons

By their properties and their particular granular structure, chondrites give us a precise clue as to their origins. Researchers have divided them into three main families:
  • Carbonaceous.
  • The ordinary.
  • Enstatites.

Ordinary chondrites, the most common, contain both volatile elements (gasgas) and oxidized elements (iron, magnesium, etc.) which have undergone rapid crystallization, sometimes in liquid-liquid form. Their spherical shape also suggests very low or no gravity. These processes suggest that they stayed inside the asteroid belt and evolved in an environment apart from planets, far from gravitational constraints.

Enstatite chondrites have abundant refractory refractory elements, a sign of high temperatures. They have been completely reduced. They would therefore come from the inner region of the Solar System.

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As for the carbonaceous chondrites, they present the most volatile elements and are also the most oxidized rocks. They would have been formed at greater distances from the Sun.

Each category of chondrite is then subdivided into subgroups to distinguish certain structures typical of their evolution.

Saint-Sauveur (Haute-Garonne) France an enstatite meteorite, Chondrite group EH5, 14Kg. Fell in 1914. Muséum de Toulouse, CC BY-SA 4.0, via Wikimedia Commons

Carbonaceous chondrites are subdivided into three main categories according to their organic matter content:
  • type C1 contains sulphates, iron magnetitemagnetite, nickel and silicates in amorphous hydrated form. Their granular structure proves that the material agglomerated at low temperature, below 500°C. These meteorites contain up to 6% organic compounds (carboncarbon);
  • type C2 contains sulfur and iron in hydrated form. These meteorites contain chondrules, small inclusions about 1 cm in diameter. They contain 2.5% carbon;
  • type C3 is less hydrated than type C2 chondrites and contains much less carbon (0.5%).

Finally, the carbonaceous chondrites are divided into various petrographic groups:

CI: Ivuna.
CK: Karoonda.
CM: Mighei.
CO: Ornans.
CR: Renazzo.
CV: Vigarano.

Nasa DART Mission | First time crashed into an asteroid to change its trajectory

Sources: PinterPandai, The Natural History Museum – London, National Geographic, Britannica

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