Geology Reference
In-Depth Information
Ureilite
5316 g
Found December 28, 1982
22.0 × 16.0 × 9.0 cm
Weathering = A/Be
Ureilites are coarse-grained ultramafic rocks containing olivine and
pigeonite. graphite is also present in many ureilites, and the Fe-Mg-o
relation in this group of meteorites indicates equilibration at high temper-
atures in the presence of C at some pressure (50 bar) within an asteroid-
sized body. Because ureilites also define an unusual oxygen isotopic
composition that has a mass-independent slope of 1:1 on a three-isotope
diagram for oxygen, clues to their origin lie in an explanation that includes
silicates, graphite, and oxygen.
0.8
Cr
2
O
3
0.6
MnO
0.4
CaO
0.2
20
10
4
2.5
1
Fa
10
0.0
PCA82506
0
60
40
20
0
Core
Distance,
µ
m
Rim
Plate 45
MiNERALogy
SigNiFiCANCE
PCA 82506 is composed of an aggregate of anhedral to
subhedral grains (0.6-3 mm in diameter) of olivine (Fa
21
),
high Cao content (0.3%), and pyroxene (Wo
6
Fs
18
En
76
).
individual grains are rimmed by carbonaceous material,
which includes thin stringers of troilite. Trace amounts of
nickel-iron metal originally present have been largely
weathered to limonite. Some grains show undulose
extinction, but apart from that the meteorite appears to
be relatively unshocked compared to most ureilites.
Along with ALH A77257, PCA 82506 is a large ureilite
collected in the early years of the program. As such, it has
influenced many systematic studies of ureilites, and its
large size has allowed wide distribution within the science
community. Cooling rate studies of PCA 82506 olivines
showed that ureilites likely cooled rapidly (right, [320]),
consistent with the idea that ureilites formed during a
disruptive impact of a large parent body.
References [312-329]
