Abstract: The crystal structure, including hydrogen positions, of borcarite, Ca₄Mg[B₄O₆(OH)₆](CO₃)₂, monoclinic, a = 17.840(4), b = 8.380(2), c = 4.445(1) Å, β = 102.04(3)°, V = 649.9(3) ų, space group C2/m, has been refined by full-matrix least-squares methods to R = 2.5% and wR = 3.0% for 1020 unique observed [|F| ≥ 5σ(F)] reflections collected using Mo-Kα X-radiation. The H positions were located on difference-Fourier maps and were refined using the ‘soft’ constraint that O-H distances are ∼ 0.96 Å. The 4:4T FBB (fundamental building block) of the borcarite structure contains four Bφ4 tetrahedra (φ = unspecified ligand) which share corners to form a four-membered polyhedral ring. Borcarite is the only mineral known to contain this FBB. The FBBs do not polymerize, but each shares three anions with an Mgφ₆ octahedron on either side, forming rods of composition [MgB₄O₆(OH)₆]^4- along the c-axis. The rigidity of these rods is enhanced by hydrogen bonding, and individual rods are connected through Caφ₈ polyhedra, Cφ₃ triangles and hydrogen bonds.