05 Classification of.. - Department of Earth and Planetary Sciences

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Classification of Meteorites
1.05.5 MARTIAN (SNC) METEORITES 116
1.05.5.1 Shergottites 116
1.05.5.2 Nakhlites (Clinopyroxenites/Wehrlites) 118
1.05.5.3 Chassignite (Dunite) 118
1.05.5.4 Allan Hills 84001 (Orthopyroxenite) 120
1.05.6 LUNAR METEORITES 120
ACKNOWLEDGMENTS 120
REFERENCES 120
1.05.1 INTRODUCTION
Solid bodies of extraterrestrial material that
penetrate the atmosphere and reach the Earth’s
surface are called meteorites. Other extraterrestrial
materials include micrometer-sized interplanetary
dust particles (IDPs) collected in the lower stratosphere
and polar ices. Most meteorites and IDPs
are fragments of asteroids, but some IDPs may
represent cometary material and some meteorites
are fragments of the planets Mars and Earth’s
moon. Meteorites recovered following observed
falls are called falls; those which cannot definitely
be associated with observed falls are called finds.
Meteorites are given names based on the location
where they were recovered (e.g., the Allende
meteorite fell in Allende, Mexico). Meteorites
recovered in Antarctica and the deserts of Australia
and northern Africa are given names and numbers,
because numerous samples are found in the same
locations. Fragments thought to be of the same
meteorite fall, which, in Antarctica or hot deserts,
may have different numbers or even names because
they were found in different locations, are called
paired. More than 1.7 £ 10 4 meteorites are
currently known; this number is rapidly growing
through the discovery of large concentrations of
meteorites in the cold and hot deserts.
Based on their bulk compositions and textures,
meteorites can be divided into two major categories,
chondrites and nonchondritic meteorites;
the latter include the primitive achondrites and
igneously differentiated meteorites (Figure 1).
They are further classified into groups using a
classification scheme based on their oxygen
isotopes, chemistry, mineralogy, and petrography
(Table 1 and Figure 1). The goals of this
classification scheme are to provide descriptive
labels for classes of meteorites with similar origins
or formation histories that could be derived from
the same asteroidal or planetary body, and to reveal
possible genetic links between various classes. In
this chapter, we describe the existing meteorite
classification, classification parameters, and general
mineralogical and geochemical characteristics
of the meteorite groups. Classification of IDPs and
meteorite–asteroid connections are only briefly
mentioned; interested readers can find additional
Figure 1
Classification of meteorites.