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

abundances and chondrule sizes by comparison to
Kakangari/Lewis Cliff 87232 (33 vol.%
versus 70–77 vol.% and 1.0–1.25 mm versus
0.25–0.5 mm, respectively). CAIs in Kakangari
and Lea County 002 appear to be different as well
(Prinz et al., 1991a). Mineralogical observations
(Krot, unpublished) and oxygen isotopic compositions
(Figure 4) suggest instead that Lea County
002 is a CR chondrite. For more on IDPs, see
Chapter 1.26.
1.05.2.4.5 R (Rumuruti-like) chondrites
The R chondrites (Figures 16(d)) are characterized
by: (i) a high abundance of matrix
(,50 vol.%); (ii) high oxidation states, reflected
in the occurrence of abundant NiO-bearing olivine
with Fa 37 – 40 (in equilibrated R chondrites) and
nearly complete absence of FeNi-metal;
(iii) refractory lithophile and moderately volatile
element abundances similar to those in ordinary
chondrites (,0.95 £ CI); (iv) absence of significant
depletions in manganese and sodium and
enrichment in volatile elements such as gallium,
selenium, sulfur, and zinc, which distinguish R
chondrites from ordinary chondrites (Figure 2);
(v) whole-rock D 17 O values higher than for any
other chondrite group (Figure 4); and (vi) extreme
rarity of CAIs (Weisberg et al., 1991; Bischoff
et al., 1994; Rubin and Kallemeyn, 1994; Schulze
et al., 1994; Kallemeyn et al., 1996; Russell, 1998).
Most R chondrites are metamorphosed (petrologic
type .3.6) and brecciated (Figure 15(d));
they have typical light/dark structure and solarwind
implanted rare gases and are best described
as R3–6 regolith breccias (Bischoff, 2000).
1.05.3 CLASSIFICATION OF IDPS
Based on the bulk chemistry, IDPs are divided
into two groups: (i) micrometer-sized chondritic
particles and (ii) micrometer-sized nonchondritic
particles. A particle is defined as chondritic when
magnesium, aluminum, silicon, sulfur, calcium,
titanium, chromium, manganese, iron, and nickel
occur in relative proportions similar (within a
factor of 2) to their solar element abundances, as
represented by the CI carbonaceous chondrite
composition (Brownlee et al., 1976). Chondritic
IDPs differ significantly in form and texture from
the components of known carbonaceous chondrite
groups and are highly enriched in carbon relative
to the most carbon-rich CI carbonaceous chondrites
(Rietmeijer, 1992; Thomas et al., 1996;
Rietmeijer, 1998, 2002).
The chondritic IDPs occur as aggregate particles
(make up ,90% of chondritic IDPs)
and nonaggregate particles. A subdivision of
chondritic IDPs includes: (i) particles that form
Classification of Nonchondritic Meteorites 103
highly porous (chondritic porous (CP) IDPs) and
somewhat porous aggregates (chondritic filled
(CF) IDPs); (ii) smooth particles (chondritic
smooth (CS) IDPs); and (iii) compact particles
with a rough surface (chondritic rough (CR) IDPs)
(MacKinnon and Rietmeijer, 1987; Rietmeijer and
Warren, 1994; Rietmeijer, 1998, 2002).
Three groups of chondritic IDPs have
been distinguished using infrared spectroscopy:
(i) olivine-rich particles þ carbon phases;
(ii) pyroxene-rich particles þ carbon phases; and
(iii) layered lattice silicate-rich particles þ carbon
phases (Sandford and Walker, 1985; MacKinnon
and Rietmeijer, 1987). The layered lattice silicaterich
particles are further divided into serpentinerich
and smectite-rich particles (Rietmeijer, 1998).
In an attempt to reconcile various chondritic
IDP classifications, Rietmeijer (1994) proposed
to classify IDPs on the basis of properties of
identifiable textural entities in these particles and
their porosity as aggregate IDPs and collapsed
aggregate IDPs. In highly porous aggregates the
constituents are loosely bound (CP IDPs). Most
aggregates are more compact with little pore
space (CF IDPs). The collapsed aggregate particles
typically have a smooth surface (CS IDPs).
The nonchondritic IDPs consist mostly
of magnesium-rich olivines and pyroxenes,
and FeNi-sulfides (Rietmeijer, 1998). Chondritic
materials are often attached to the surface
of nonchondritic IDPs, which suggests that the
groups have a common origin. For more on IDPs,
see Chapter 1.26.
1.05.4 CLASSIFICATION OF
NONCHONDRITIC METEORITES
The nonchondritic meteorites contain virtually
none of the components found in chondrites
and were derived from chondritic materials by
planetary melting, and fractionation caused their
bulk compositions to deviate to various degrees
from chondritic materials. The degrees of melting
that these rocks experienced are highly variable
and, thus, these meteorites can be divided into two
major categories: primitive and differentiated
(Figure 1). There is no clear-cut boundary
between these categories, however.
The differentiated meteorites were derived from
parent bodies that experienced large-scale partial
melting, isotopic homogenization (ureilites are the
only exception), and subsequent differentiation.
Based on abundance of FeNi-metal, these meteorites
are commonly divided into three types:
achondrites (metal-poor), stony irons, and irons;
each of the types contains several meteorite
groups and ungrouped members.
The primitive nonchondritic meteorites have
approximately chondritic bulk compositions, but