Chemistry Reference
In-Depth Information
16.6 CARBYNES IN METEORITES
The primitive carbonaceous meteorites, which include the hydrated CI and
CM meteorites and mostly-anhydrous meteorites such as the Allende CV
meteorite [69], reach Earth from the asteroid belt between Mars and Jupiter.
Asteroid reflectance properties display a remarkably systematic distribution
as a function of heliocentric distance for asteroids in this belt, and hence
meteorite types, with the most primitive ones located farthest from the sun.
Asteroid hydration occurred when internal heating melted (water) ice that
had co-accreted with dust, chondrules and refractory inclusions in the solar
nebula. These asteroids form the IR spectroscopic C-class with ''clays,
carbon and organics'' at the surface similar to CI and CM meteorite parent
bodies [70]. They and the Allende CV parent body, which apparently did not
accrete (much) ice, are from the same zone of the asteroid belt. Even more
primitive asteroids closer to Jupiter still contain co-accreted ices, organic
materials and silicate dust. They define the IR spectroscopic primitive (P)-
and dark (D)-class [70] bodies that include comet nuclei and many near-
Earth asteroids [10].
Non-volatile materials in asteroids included 1. presolar (interstellar) dust,
and 2. dust condensed from a chondritic vapor in the inner solar system
where presolar silicate and carbon dusts had evaporated at
2000K during
a thermal flare-up phase of the young Sun [71]. Evidence for both dust
sources can be found in carbonaceous meteorites albeit they were modified
by aqueous alteration below
800K in
their parent bodies [69,72,73]. When metastable carbynes were not oblit-
erated by parent body alteration, these particular meteorites would be
candidates for searches of carbynes formed (probably by condensation)
around C-rich YSOs (interstellar dust), in the inner solar system, or both.
In the quest to find the carrier phases of interstellar stable-isotope
signatures, a volume of carbonaceous meteorite was dissolved in HCl-HF
and concentrated in an acid-resistant residue of carbonaceous matter, spinel
(MgAl
2
O
4
), chromite (FeCr
2
O
4
) and hibonite (CaAl
12
O
19
). Such residues
of the Allende and Murchison meteorites reportedly contained single-
crystal grains of carbon VI, -VIII, -X, -XI, -XII and -IX [21,74]. Carbon
XII (maximum d-spacing: 0.453 nm) in the Murchison residue might be
a
-carbyne or chaoite, (110)
400K, or thermal alteration up to
0.447 nm [21]. The identifications were mostly
based on the maximum d-value in single-crystal c-axis electron diffraction
patterns. There is no quantitative data information on individual carbyne
abundances in the residues. Irregular angular grains were
ΒΌ
40 to 750 nm in
size with a conchoidal (glassy) fracture surface in the Allende sample but
they were porous with a ''loose structure'' in the Murchison sample [21].
Typical faceted, i.e. euhedral hexagonal, platy carbyne crystals were rare.
Some became quickly amorphized when placed in the incident TEM
electron beam. The metastable nature itself was used to distinguish different
carbynes. Of two crystals with near identical maximum spacing (0.464;
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