PDF (Hi-Res) - Smithsonian Institution Libraries

76 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 3.—Photomicrograph of thin section of the Kuttippuram
L6 veined white chondrite. Note the "achondritic"
fragment, lower half, consisting mostly of olivine and glass,
and almost devoid of metal and troilite. The olivine-pyroxene
composition in the fragment is the same as in matrix and
chondrules. Length of section, 3 mm.
sthene chondrite, the variable olivine-pyroxene
composition and especially Figures 4 and 5 illustrate
why the classification is exceedingly difficult.
Our sample of Mabwe-Khoywa is a complex rock, a
breccia or conglomerate. In addition to a great
variety of chondrules it contains lithic fragments,
rounded or angular; some of these were pre-existing
chondrites, while others appear to be achondritic.
Most olivines and pyroxenes, 90%-95% in these
parts, with the exceptions described below, still are
in the dominant range, as are numerous individual
larger mineral fragments. The main part of the
matrix also consists of comminuted chondrules and
chondrite fragments. In general, but not always,
olivines in the matrix seem to be richer in iron than
olivines in fragments, while the lowest iron content
is found in some rare chondrules (Figure 5). Also,
the larger olivines in the "achondritic" fragments
are more iron rich, similar to the dominant olivine,
than the smaller grains. At least three types of
pyroxene were found. The dominant orthopyroxene
is found in almost all types of chondrules and fragments
and is essentially homogeneous, i.e., equilibrated.
The diopsidic pyroxene varies at least from
Wo20 to Wo40 with corresponding variations in iron
and magnesium. The third pyroxene, approximately
enstatite, is rare and was only found in
random point analyses (several hundreds) and,we
could not establish in which kind of fragment it
occurs. A few pyroxenes in the range between the
orthopyroxene and the diopside could represent
overlap between host crystals and exsolution
lamellae.
Some of the chondrules and fragments in Figure
4 deserve brief individual descriptions. In the
olivine-pyroxene chondrule (Figure 5), the olivine
has the lowest iron content encountered, and most
crystals are zoned from Fa8 in the center to Fa10 at
the edge. The pyroxene appears to be monoclinic
and has exsolution lamellae, but nearly constant
composition (^Woj En87 Fs12); thus the nonparallel
extinction and the exsolution lamellae are probably
shock produced. Further indication of shock effects
is the opaque spherule consisting of apparently
quenched eutectic troilite plus nickel-iron similar
to those in the H-chondrite Pulsora (Fredriksson,
et al., 1975).
Adjacent to the chondrule is a chondritic fragment
containing a fragment of a barred olivine and
glass chondrule. The olivine bars (Figure 5) seem
intergrown with the first chondrule but belong to
the equilibrated main type, Fa^,24. Also other
olivines and pyroxenes in this chondrite fragment
belong to the main type. The glass in the barred
chondrule is almost albitic but has slight normative
excesses of quartz and corundum.
Figure 4 shows a large "achondritic" fragment
consisting largely of olivine and glass with metal,
troilite, and chondrules conspicuously lacking. The
smaller olivine crystals have low iron content,
Fa^,12, while the larger crystals are somewhat zoned
but close in composition (Fa20_22) to the "equilibrated"
main olivine. This might indicate that the
larger crystals are relatively late xenoliths. The
interstitial glass is enriched in silica (^62%) and
alumina (^8%), but has low soda (^0.8%) and
substantial amounts of iron (16% FeO) and magnesia
(