11. The ?-polymorph consists of black hexagonal crystals , which turn dark green when exposed to air. 12. The species Cs 2 O forms yellow-orange hexagonal crystals . 13. The Bravais lattices in the hexagonal crystal family can also be described by rhombohedral axes. 14. It crystallizes in the hexagonal crystal system typically occurring as radiating acicular prisms and massive encrustations. 15. They form when beams kf0qunlight pass though hexagonal crystals of ice that are in cirrus clouds. 16. But not all hexagonal crystals are trigonal .) 17. It has a hexagonal crystal structure, with layers probably stacked in the sequence AcBiAbCjA . .. 18. It is not known whether Quaoar's ice consists of hexagonal crystals or an alternative cubic structure. 19. Trigonal, tetragonal and hexagonal crystals have a single optic axis, parallel to the c crystal axis. 20. It crystallizes in the hexagonal crystal system and has been found as primary euhedral di-rhombohedral, pseudo-octahedral crystals.
