The stoichiometry of boron suboxide (B 6 O 1؊ ) synthesized from mixtures of boron and boron oxide (B 2 O 3 ) at high pressure lies closer to the nominal composition ( ؍ 0) than materials obtained at atmospheric pressure. The materials obtained in the high pressure syntheses in the presence of molten B 2 O 3 also have a higher degree of crystallinity than for sintered powders. For syntheses at temperatures of 1700}18003C at pressures between 4 and approximately 5.5 GPa, the well-crystallized particles are dominated by icosahedral multiply-twinned particles up to approximately 40 m in diameter. This unusual morphology is obtained by Mackay packing, i.e., by assembly of successive shells of icosahedral B 12 units around a central icosahedral nucleus to give a multiply twinned particle in which each of the 20 elements has the R3 m space group of the rhombohedral -B structure. We examine the thermodynamic and kinetic factors associated with the development of this morphology during high pressure growth and use ab initio calculations to investigate the energetic driving forces for initiation of the Mackay packing around the central icosahedral nucleus.