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main source of dust in the ISM. Eventually, such ejected dust will form a
circumstellar envelope, which finally gives rise to a planetary nebula that
with time is dispersed as part of a continuous process of dust formation and
recycling.
Carbon chains would thus be produced by carbon-rich stars and
subsequently expelled into the ISM included in other dust grains. In the
millions of years between circumstellar dust formation and the appearance
of carbon chains in cold interstellar molecular clouds such chain molecules
are unlikely to survive photodissociation when not protected by carbon
grains. There remains a remote possibility that polyyne molecules associated
with interstellar grains could yield carbyne crystal grains when interacting
with supernova-generated shock waves via carbon condensation when
precursor molecules were evaporated. Considering that the pressure for
grain shattering in grain-grain collision is on the order 10
10
Pa, which is
considerably lower than the pressure for grain vaporization (5
10
11
Pa)
[111], there could be another potential pathway for interstellar carbyne
crystal formation. The x-ray pattern of graphite produced by arc discharge
of a CH
4
/CO mixture samples treated at 10
9
Pa pressure showed maxima
that were attributed to metastable chaoite suggesting it had a modified
graphite structure rather than a chain structure [112], which would be
consistent with the new carbyne model Perhaps the high
collision energies in the cold ISM could produce a kinetically controlled
environment wherein linear C
6
molecules included in carbon grains would
not to covert to the cyclic form but rather form benzene rings. Smaller linear
carbyne molecules could then connect two such rings to form a carbyne
(
Figure 16.5
).
crystal (see
Section 16.10
)
. However, when carbynes crystals are not formed
in the ISM but only sojourn through the ISM to developing circumstellar
dust environments, the linear carbon molecules have to be locked within
amorphous carbon grains. These carbon grains might resemble the amor-
phous carbon-nanodiamond-carbyne association in the Murchison meteor-
ite [20]. The conjectural nature of the sections dealing with carbyne crystals
among the dust in circumstellar and interstellar environments in part reflects
the inability to uniquely constrain the exact form of solid carbons by remote
sensing techniques.
16.13 CONCLUSIONS
This review of natural carbynes took a mineralogical approach by accepting
carbyne crystals only as metastable forms of purely elemental carbon.
Carbynes appear to be almost exclusively produced synthetically as the
condensation products of carbon vapors but carbon vapors are not part of
the geological processes on Earth. Natural terrestrial carbynes, with chaoite
as the preferred natural carbyne, only occur as lamellae in graphite crystals
superheated in dynamic environments characterized by the catastrophic
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