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3. Readily suffer decomposition, vesiculation and amorphization when
exposed to an incident energy beam, e.g. during TEM analyses,
laboratory heat-treatment or irradiation [14,19,21].
0.7 Myrs old
Ries impact crater [22] and in 60-Myrs old Ticonderoga marble [23] were
taken to support the suggestion that chaoite could be a stable natural
carbyne [24]. This argument of geologic age tacitly assumes continuous
change of mineral properties as a function of ''geological time''. Yet,
minerals commonly persist outside their thermodynamic stability field as
long as there are no activation energies to initiate a reaction. When they are
not thermodynamically stable phases, are natural carbynes real minerals? Is
chaoite, listed as a mineral by the Committee on New Minerals and Mineral
Names of the International Mineralogical Association, the only exception?
According to the Glossary of Geology [2], a mineral is a naturally occur-
ring element or chemical compound, usually an inorganic, crystalline sub-
stance, with characteristic physical and chemical properties that are due to
its atomic arrangement. The keywords are ''naturally occurring'', meaning a
thermodynamically stable synthetic crystal cannot be a mineral until it is
identified in a natural environment with distinct and unique P,T constraints.
With this mineral definition, metastable natural carbynes are not minerals
and consequently they are not carbon allotropes. Arguments counter to this
mineralogical position exist [25].
The ages of chaoite lamellae in graphite from the 14.7
16.3 THE CARBON PHASE DIAGRAM REVISITED:
CARBON ''MELTING''
The carbon phase diagram has been modified to accommodate a growing
database on metastable carbons [13,26]. It is germane to recall that pure
carbon solid reactions will be diffusion-less transformations that occur at
the speed of sound. For example, a martensitic transformation (polysyn-
thetic twinning) or a massive transformation (transgression of the new phase
across pre-exiting grain boundaries); both will be nearly instantaneous at
submicron scales. Metastable carbons will respond rapidly to kinetically
controlled energy dissipation. One modified version phase diagram includes
a high-T/low-P ''carbynes'' field [27]. This modification presumed that
carbynes are equilibrium phases on a par with (hexagonal) graphite and
(cubic) diamond. These only true carbon allotropes define a hexagonal
graphite
cubic diamond phase boundary and allow for reversible meta-
stable conversions more or less along the metastable extension of the
diamond-melting curve [26,28]. This led to the proposal of a ''vapor-like
intermediate'' state wherein carbon atoms are disorganized in this meta-
stable region of the phase diagram [29]. We submit an intermediate solid-
carbon phase in the formation of metastable carbon solids to bring perhaps
,
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