Techniques d'observation spectroscopique d'astéroïdes

CHAPTER 4. SPECTRAL ANALYSIS TECHNIQUES 69
• meteorites that contain comparable amounts of macroscopic metallic and rocky components
are called stony irons.
• meteorites that do not contain large concentrations of metal are know as stones.
tel-00785991, version 1 - 7 Feb 2013
A second classification of meteorites takes into account their mineralogic changes: achondrites
are igneous bodies, the product of melting, changes in composition and recrystallization,
while chondrites are the primitive meteorites composed of material that formed the solar nebula
and surviving interstellar grains, little modified in some case by aqueous and/or thermal
processes.
Chondrites keep the records of the origin and the early evolution of the Sun and planets. The
name comes from Greek - "chondros", meaning grain or seed, a reference to the appearance
produced by numerous small, rounded inclusions called chondrules. These chondrules are
small droplets of olivine and pyroxene condensed and crystallized from the hot primordial
solar nebula in form of small spheres. They accreted with other material that condensed from
the solar nebula forming a matrix [McSween, 1999]. The chondritic meteorites are split in:
ordinary chondrites so named because they are the most abundant type; the carbonaceous
chondrites actually misnamed when they were believed to have much higher carbon contents
than other chondrites; the enstatite chondrites named for their high abundances of enstatite, a
magnesium silicate mineral. Rumuruti and Kakangari chondrite meteorites does not fit in any
of these classes, being considered separate types [de Pater & Lissauer, 2010].
The most common primitive meteorites - ordinary chondrites are divided based on their
Fe/Si ratio: H - high Fe content, L low Fe, and LL low Fe and low metal. The same criterium
applies for enstatite chondrites EH, EL. The carbonaceous chondrites are split in eight classes
which slightly differ in composition: CI,CM, CO, CV, CR, CH, CB, and CK [de Pater & Lissauer,
2010].
In Fig. 4.2a is given the microscopic view of a thin section of the Allende meteorite(CV3).
It shows numerous chondrules, white calcium-aluminum inclusions (CAI), and opaque metal
grains, all held together by dark, fine-grained matrix material. All of this diversity is contained
within several square centimeters of surface area in this meteorite. For comparison, the
abundance of elements in the Sun’s photosphere is plotted against their abundance in the carbonaceous
chondrites [Ringwood, 1979]. Most elements lie very close to the curve of equal
abundance (normalized to Si).
Chondritic meteorites are assigned a petrographic type ranging from 1 to 7. This describes
the degree of alteration by different processes. Type 3 chondrites appear to be least altered and
provide the best data on the conditions within the protoplaneatry disc. Types 5 to 7 are shocked
materials, signature of collisional processes of parent bodies.
In contrast to chondrite, achondrites came from differentiated parent bodies (bodies that
have undergone density-dependent phase separation). The igneous origin of these meteorites
imply a partial or total melting of primordial chondritic matter. Igneous processes are the