Natural Carbynes, including chaoite, on Earth, in meteorites, comets, circumstellar and interstellar dust

This paper on natural carbynes adopts a strictly mineralogical approach that will be necessary to appreciate the information available on these peculiar carbons. Ternary classification of elemental carbons using the sp-hybridized carbon bond character, sp3 (cubic diamond), sp2 (hexagonal graphite) and sp1 (linear carbynes), creates ‘‘chemical’’ order [1] to focus discussions. It presumes equality among elemental carbon solids in a mineralogical 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 techniques with poor instrumental resolution. For example, the geological term ‘‘cryptocrystalline’’ refers to a natural volcanic glass that viewed in a lightoptical 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) spectroscopic and HRTEMinformation from solid carbon analogs as a proxy of processes in astronomical environments [4,5]. In these environments, amorphous 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.