NWA xxx unranked with a good form. Chondrite Meteorite - 133 g - (1)

Chondrites are the most common type of meteorites (85%). They are fragments of small asteroids, ejected from the main belt by a collision-impact between two asteroids. The main photograph shows a polished section of an ordinary chondrite (H chondrite) found in Morocco.

Composition of a chondrite. A chondrite is composed of chondrules, embedded in a matrix. The chondrules (c) are small spheres resulting from the rapid solidification of microdroplets of a slightly ferrous silicate liquid, which quickly solidify into a glassy bead (now recrystallized). The matrix consists of an aggregate of small silicate crystals (s) that are brownish to greenish (olivine and pyroxene, more or less hydrated, thus very partially serpentined and/or clayed), with metallic iron (f) trapped between the silicate crystals. The chemistry of the chondrules is similar to that of the matrix silicates. Overall, a chondrite (chondrule + matrix) contains about 20-30% metallic iron, and 70-80% silicates. The silicate mixture (density 3.3) and iron (density 8) shows that this mixture has never melted after its formation; otherwise, the density difference would have caused separation, differentiation, between the silicate part and the metallic part. This chemistry exactly matches that of the refractory elements of the Sun (obtained through spectral studies). Furthermore, the chemistry of the Earth's mantle and crust is relatively well known; the chemistry of the core is much more speculative, but seismic data combined with experimental measurements strongly suggest that the core is made of at least 80% metallic iron. By summing the Earth's crust + mantle + core (with their relative proportions), a theoretical chemical composition of Earth can be derived (with uncertainties corresponding to the uncertainties in the core). The composition of chondrites is close (within uncertainties) to this theoretical composition of Earth. Assuming that chondrites and Earth have exactly the same composition, it is then possible to calculate the precise composition of the Earth's core by subtracting the elements of the mantle + crust from those of a chondrite.

Origin of chondrites. During the condensation of the nebula, dust particles of iron and silicates 'floated' between 0.5 and 3 Astronomical Units from the Sun. A very poorly understood phenomenon led to the formation of chondrules, small droplets of liquid that quickly solidified. Another, also poorly understood, process gathered chondrules and silicate-iron dust into bodies measuring hundreds of meters to kilometers in size: the 'planetesimals.' Then, gravity came into play, and the planetesimals accreted into increasingly larger bodies, such as asteroids and planets. The heat generated by accretion caused melting, differentiation (by gravity), and the separation of core and mantle in the larger bodies (like Earth), but not in the smaller ones. Chondrites originate from asteroids that were too small to have been melted. They were recently torn from parent bodies (small asteroids) by inter-asteroid collisions somewhere in the asteroid belt.