Natural Carbynes,Including Chaoite, on Earth, in Meteorites, Comets, Circumstellar and Interstellar Dust - Polyynes: Synthesis, Properties, and Applications - page 340

stresses over geological short time (<1m.y.) along major fault zones could
cause thermal spikes in the thermal regime of regional metamorphism but
such thermal spikes are modest at <1000K [56,57]. Such frictional shear
heating in regionally metamorphic rocks was proposed for the graphitiza-
tion of anthracite [58]. Higher transient temperatures were inferred for the
formation of pseudotachylites, rocks that resemble volcanic glass. These
rocks are formed during shear heating along tectonic fault movements
when crustal rocks are melted by shear-stress release, although the exact
temperatures are unknown. We submit that depending on the amount of
shear-stress energy built-up and the rates of catastrophic heat release and
dissipation during an earthquake on a tectonic fault line could spike

temperatures high enough to ''melt'' graphite in the EDAC field ( Figure
16.2 ) . Here too, graphite expansion will be more dominant than compres-

sion during solid-state transformation of graphite to thin a -carbyne or
carbon VIII lamellae when two adjacent, thermally expanded (10-15%)
graphite layers ''melted'' in the EDAC field to match carbon VIII, c 0 ¼
1.482 nm, and a -carbyne, c 0 ¼

1.536 nm. The required percentage of linear
graphite expansion is very high and unrealistic considering the general
equilibrium temperatures during regional metamorphism. Graphite super-
heating would be possible during catastrophic shear-stress release in an
earthquake event. The co-occurrence of a - and b -carbynes in the marbles
[23] supports geologic annealing of metastable a -carbyne to more stable,
denser b -carbyne, which is consistent with results of thermobaric treatment
(5min; 2073K; 9GPa) of a -carbyne [19].
Chaoite, a -carbyne and carbon VIII lamellae in natural graphite are a
unique indicator for heat dissipation from catastrophic shear energy release
in hypervelocity shock events and during earthquake events on regional
geologic scale. The carbynes formed during kinetically controlled solid-state
transformations from a ''melted'' carbon phase in the EDAC field. To the
best of our knowledge this is the only natural carbyne-forming process
on Earth. The suggestion that chaoite in the Ries crater was unrelated to
the impact event [23] is not supported. There are other ways to produce
carbynes, e.g. chaoite via graphite pyrolysis at high temperature from 2600
to 3000K in a low-pressure (0.01 Pa) argon atmosphere [59] but such con-
ditions were not yet observed in natural terrestrial environments.

16.5 CARBYNE IDENTIFICATION

Carbyne identification is still a matter of contention since not all identi-
fications are complete and rigorous. They are generally identified by color,
hardness, fracture surface, density and XRD, a bulk technique that
produces a list of interplanar (hkl)ord-values [23]. Subtle combinations
of d-values and x-ray line intensity are used for carbyne identification [23]
but this approach will fail when using electron diffraction data with an up to