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

90
Classification of Meteorites
Table 2 Summary of the average petrographic properties of the chondritic meteorites.
Carbonaceous Ordinary Enstatite Additional
CI CM CO CR CH CB CV CK H L LL EH EL K R
CAI þ AOA (vol.%) p1 5 13 0.5 0.1 ,0.1 10 4 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1
chd (vol.%) p1 20 48 50–60 ,70 30–40 45 15 60–80 60–80 60–80 60–80 60–80 27 .40
matrix (vol.%) .99 70 34 30–50 5 ,5 40 75 10–15 10–15 10–15 ,0.1 ,0.1 60 30
metal (vol. %) 0 0.1 1–5 5–8 20 60–70 0–5 ,0.01 8.4 4.1 2.0 10.1 10.2 7.4 ,0.1
chd, mean diam. mm 0.3 0.15 0.7 0.02 0.1–20 1 0.7 0.3 0.7 0.9 0.2 0.6 0.6 0.4
as type 7, although some may be impact and not
internally derived melts.
The type-3 ordinary, CO, and CV chondrites are
commonly subdivided into 10 subtypes (3.0–3.9),
of which 3.0 is the least metamorphosed (e.g.,
Sears et al., 1991). Table 3 provides a summary
of the criteria used to define petrologic types.
1.05.2.3.2 Shock metamorphism stages
The degree of shock metamorphism (caused by
impacts) recorded in a chondrite is determined
from a variety of mineralogical and textural
parameters (e.g., Stöffler et al., 1991; Scott et al.,
1992). The classification scheme by Stöffler et al.
(1991) is based on shock effects observed in
olivine and plagioclase (Table 4). Since olivine is
rare in enstatite chondrites, Rubin et al. (1997)
extended this shock classification scheme to
orthopyroxene (Table 4).
1.05.2.3.3 Classification of breccias
In addition to shock metamorphism, impacts
between solar-system objects (asteroids and
comets) result in formation of breccias—rocks
composed of fragments derived from previous
generations of rocks, cemented together to form a
new lithology. Many chondrites are breccias.
A breccia in which the clasts belong to the same
group of meteorites is called genomict. A breccia
in which the clasts and/or matrix belong to
different meteorite groups is called polymict.
The classification of chondritic breccias is summarized
in Table 5.
1.05.2.3.4 Degree of terrestrial weathering
Degree of terrestrial weathering is an additional
classification parameter commonly applied to
meteorite finds. Two classification schemes are
used: one for hand specimens of Antarctic
meteorites (commonly used), and one for meteorites
as they appear in polished sections (rarely
used). Weathering categories for hand specimens
are: A—minor rustiness; B—moderate rustiness;
C—severe rustiness; and e—evaporite minerals
visible to the naked eye (e.g., Grossman, 1994).
Wlotzka (1993) suggested the following progressive
alteration stages for meteorites as they
appear in polished sections: W0—no visible
oxidation of metal or sulfides; W1—minor oxide
veins and rims around metal and troilite;
W2—moderate oxidation of ,20–60% of metal;
W3—heavy oxidation of metal and troilite,
60–95% being replaced; W4—complete oxidation
of metal and troilite, but no oxidation of silicates;
W5—beginning alteration of mafic silicates,