How many crystalline forms of pure boron are known?

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A labeled diagram of the periodic table entry for boron shows its atomic number 5, symbol B, electron configuration, atomic weight range, and classification as a solid, weakly acidic, other nonmetal with a rhombohedral crystal structure at room temperature. — boron Properties of boron.

Pure boron exists in at least four known crystalline modifications, or allotropes. These forms are characterized by the presence of closed cages containing 12 boron atoms arranged as an icosahedron.12

According to data on the properties of boron group elements, the identified crystalline structures at 20 °C include:

Alpha-rhombohedral
Beta-rhombohedral
Tetragonal3

While these represent the primary structural categories, the element is recognized to exist in at least four distinct crystalline modifications in total.1