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peculiar carbons. Ternary classification of elemental carbons using the
sp-hybridized carbon bond character, sp
3
(cubic diamond), sp
2
(hexagonal
graphite) and sp
1
(linear carbynes), creates ''chemical'' order [1] to focus
discussions. It presumes equality among elemental carbon solids in a min-
eralogical sense when, for example, considering carbon allotropy rather than
polymorphy that is, strictly speaking, incorrect. Mineralogists accept only
two carbon allotropes (hexagonal graphite and cubic diamond) each with
a thermodynamic pressure-temperature (P-T ) stability field that could
''trespass'' on each other's stability field for kinetic factors delaying an
equilibrium transformation. Hence, orthorhombic graphite, cubic graphite
(cliftonite), hexagonal diamond (lonsdaleite; carbon-2H), carbynes and
fullerenes, are metastable crystalline carbons free of heteroatoms.
Chemical and mineralogical classifications have elements of tradition left
over from the time when these disciplines used different analytical tech-
niques with poor instrumental resolution. For example, the geological term
''cryptocrystalline'' refers to a natural volcanic glass that, viewed in a light-
optical microscope, appears to be without structure, i.e. is amorphous, but
it often shows evidence for ordered entities in an x-ray diffraction (XRD)
pattern [2]. Today's high-resolution transmission electron microscopes
(HRTEM) make it possible to view the material properties at the unit-cell
level, such as the transition of a single crystal to an amorphous solid, and
vice versa. The interface between chemistry and mineralogy, emphasizing
transitions from chemical to crystalline bonds, is becoming important to
astromineralogy [3] seeking to match infrared (IR) to ultraviolet (UV) spec-
troscopic and HRTEM information from solid carbon analogs as a proxy of
processes in astronomical environments [4,5]. In these environments, amor-
phous solids quenched from a carbon liquid will be unlikely, but they cannot
a priori be dismissed. Rather highly disordered, quenched carbon-vapor
nanomaterials will be common. The combination of sp-hybridized carbon
bond information, structural properties (amorphous or crystalline), IR and
UV spectral characterization of condensed carbon nanograins obtained
in situ in the laboratory, will be an increasingly fertile area of research for
direct comparison with remote-sensing data for natural elemental carbon
solids (carbynes and fullerenes) in environments that are inaccessible to
direct sampling [6,7]. Cross-pollination of disciplines will only bear fruit
when interpretations and ideas are challenged. We accept the existence of
carbynes as metastable elemental carbon solids when assessing the evidence
for natural terrestrial and extraterrestrial carbynes.
16.2 METASTABLE CARBYNES: ARE THEY MINERALS?
A recent book entitled Carbyne and Carbynoid Structures [1] makes a strong
case that the carbyne forms of carbon sensu stricto, viz. polyynes with
conjugated acetylenic triple bonds, (-C
C
C
C-)
n
, and (poly)cumulene
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