The original hexagonal tricarbon C 3 allotrope has recently been claimed from ab initio investigations to be superdense and superhard , supporting an earlier claim for such properties . Hexagonal C 3 is derived from silica (quartz) and assigned the qtz topology (SACADA qtz . The five-ring carbon topology unj (SACADA #29) has also been proposed for hexagonal C 3 . Finally, we have recently investigated the linear C-C-C arrangements known for the isolated molecule in rhombohedral and hexagonal ultra-hard C 3 characterized by mixed sp 3 -sp 2 carbon hybridizations .