Crystals of a new rare-earth borate NaNd[B6O9(OH)4], space group \(P\bar 1\), are prepared hydrothermally. The crystal structure of the borate is solved without a priori knowledge of its chemical formula. A new complex chain radical with the formula [B 4 T B 2 Δ O9(OH)4] ∞ 4− or 6:[4T + 2Δ] is built as follows: corner-sharing BO4 tetrahedra are united by pseudoaxis 3 1 into a helical chain; the chain is lined with two BO3 triangles and one tetrahedron BO4, also linked through their corners. The varieties of this chain, including those existing in still-wellite and its synthetic analogs, correspond to four versions of the attachment of terminal polyhedra (B, P, or Si tetrahedra or B triangles) to the central helical stem of BO4 tetrahedra, thus showing the topological unity of borates, borosilicates, and borophosphates. The new hexaborate and related compounds are positioned in a new subgroup in the system, since central (stem) tetrahedra in them are multiplied by another symmetry operation than in the known hexaborates, which are built of blocks recognized in macallisterite Mg2[B6O7(OH)6]2 · 9H2O.