266 Encyclopedia of the Solar System
Chemical changes involving loss of a constituent, like
carbon or water in chondrites, require an open system;
other changes in Table 4 could occur in open or closed sys-
tems. We emphasize that thermal metamorphism can only
affect secondary (parent body) characteristics—those listed
horizontally in Table 4—not primary ones. Postaccretionary
processes by which H chondrite-like material can form from
L or vice versa are unknown.
2.4.2 CHEMICAL-PETROLOGIC CLASSIFICATION
Because properties of a given chondrite reflect both its
primary and its subsequent histories, a chondritic classi-
fication scheme reflecting both is used. Chondrites already
mentioned are Ensisheim, LL6; Nogata, L6; Sharps, H3
(Fig. 12a); Sylacauga, H4; and Kernouve and Peekskill, H6
(Fig. 12b). No ordinary (or enstatite) type 1 or 2 chondrite is
known. Type 3 ordinary chondrites, the unequilibrated or-
dinary chondrites (UOC), vary the most among themselves
and from chondrites of other petrographic types. Within
UOC, a variety of properties—for example, the chemical
heterogeneity of ferromagnesian silicates, the contents of
highly elements (mainly noble gases), and thermolumines-
cence (TL) sensitivity—subdivide UOC into subtypes 3.0
to 3.9. Sharps (Fig. 12a) is the most primitive H chondrite
known, being an H3.0 or H3.4, depending on the classifica-
tion criteria used. (A similar subclassification of CO chon-
drites also exists.)
Many properties of ordinary chondrites demonstrate
that each group has its special history, even in something as
simple as the numbers of each chemical-petrographic type
(Table 1). For example, proportions of H3 or L3 are low,
1–2% (5 of 316 H and 7 of 350 L chondrite falls), whereas
13% of LL falls (9 of 72) are LL3. Proportions of more
evolved chondrites also differ (Table 1). The plurality of H
falls are H5 (138 of 316 or 44%) while type 6 dominates L
and LL chondrites (239 of 350 or 68% and 35 of 72 or 49%,
respectively). Non-desert-cluster chondrite finds generally
exhibit similar trends. Stony-iron and, especially, iron finds
are very numerous because they are obviously “strange,”
hence more likely to be brought to someone knowledgeable
enough to identify them as meteoritic. Achondrites grossly
resemble terrestrial igneous rocks and are less likely to be
picked up: Only their fusion crust permits ready recognition
of their exotic origin (Table 1).
2.4.3 BRECCIAS
Even though most chondrites are readily pigeonholed, a
few consist of two or more meteorite types, each readily
identifiable in the lithified breccia. Noblesville, for example,
consists of light H6 clasts embedded in dark H4 matrix
(Fig. 1a). Such an assemblage—two petrographic types of
the same chondritic chemical group—is a genomict breccia.
A polymict breccia contains two or more chemically distinct
meteorite types, implying the mixing of materials from 2
(or more) parent bodies, each with its own history. The
most striking such case is Cumberland Falls where black
forsterite chondrite inclusions as large as 3 cm×5 cm are
embedded in an 8 cm×11 cm white enstatite achondrite.
Of the other sorts of breccias, perhaps the most impor-
tant is the regolith breccia. Noblesville (Fig. 1a) is such
a meteorite, and its typically dark and fine-grained ma-
trix contains large quantities of light noble gases—He and
Ne—of solar origin (cf. Section 5.1). In addition to these
gases, radiation damage in present as solar-flare tracks (lin-
ear solid-state dislocations) in a 10-nm-thick rim on the
myriad matrix crystals. However, solar gases and flare tracks
are absent in the larger, lighter-colored clasts of regolith
breccias. Clearly, dark matrix is lithified fine dust originally
dispersed on the very surface of the meter-thick regolith
or fragmental rocky debris layer produced by repeated im-
pacts on bodies with no protective atmosphere. (The lu-
nar regolith is both thicker,∼1 km, and more mature and
gardened, or better mixed by impacts than are asteroidal
regoliths.) This dust acquired its gas- and track-component
from particles with keV/nucleon energies streaming out-
ward as solar wind or solar flares with MeV energies [see
TheSolarWind] so that the dust sampled the solar pho-
tospheric composition. The irradiated dust, often quite rich
in volatile trace elements from another source, was mixed
with coarser, unirradiated pebble-like material and formed
into a breccia by mild impacts that did not heat or degas the
breccia to any great extent. Regolith breccias occur in many
meteoritic types but are especially encountered as R (and
H) chondrites, aubrites, and howardites.2.4.4 CARBONACEOUS CHONDRITES
2.4.4.1 Composition
The only type 1 or 2 chondrites are carbonaceous chon-
drites, of which nearly all non-Antarctic ones are observed
falls. A dominant process recorded in them involves hydrol-
ysis, the action of liquid water (in the nebula or on parent
bodies) that altered preexisting grains, producing various
hydrated, clay-like minerals. The chondrites’ petrography
and the decidedly nonterrestrial^2 H/^1 H ratios in water from
them show that this hydrolysis was preterrestrial. As noted
earlier, oxygen isotopic compositions of hydrated miner-
als demonstrate that the two groups derive from different
batches of nebular matter; thus, C1 (or CI) could not form
C2 (or CM) by thermal metamorphism nor could C1 have
formed by hydrolysis of C2 parent material. For this reason,
some specialists prefer the CM designation: others prefer
a hybrid classification like C2M or CM2 because other C2-
like chondrites exist. Tagish Lake, although very primitive,
is unique.
C1 chondrites containnochondrules (Table 4), but
their obvious compositional and mineralogic similarities