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In page Boron:

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Boron carbide's structure is only approximately reflected in its formula of B4C, and it shows a clear depletion of carbon from this suggested stoichiometric ratio. This is due to its very complex structure. The substance can be seen with empirical formula B12C3 (i.e., with B12 dodecahedra being a motif), but with less carbon, as the suggested C3 units are replaced with C-B-C chains, and some smaller (B6) octahedra are present as well (see the boron carbide article for structural analysis). The repeating polymer plus semi-crystalline structure of boron carbide gives it great structural strength per weight.[citation needed]