What is the cause of carbon allotropy?

Allotropy is the phenomenon of the existence of one and the same chemical element in the form of simple substances that are different in structure and properties. Carbon forms modifications that differ from each other in appearance: 1) diamond – transparent, solid, crystalline, 2) graphite – dark, gray, crystalline, has a metallic luster, 3) carbyne – black powder in the form of small crystals, 4 ) fullerene – in the form of cluster particles with a large number of carbon atoms – from 60 to 100. The reason for the existence of varieties of the simple substance carbon is in the structure of electron orbitals. They have different geometric structures and different degrees of hybridization of electron clouds. As a result, diamond, carbyne, graphite and fullerene have different crystal lattices.