Abstract: The crystal structure of KBSi₃O₈ (orthorhombic, Pnam, with a = 8.683(1), b = 9.253(1), c = 8.272(1) Å, V = 664.4(1) ų, Z = 4) has been determined by the direct method applied to 3-dimensional reflection data. The structure of a microcrystal with the dimensions 20 × 29 × 37 µm was refined to an unweighted residual of R = 0.031 using 386 non-zero structure amplitudes. KBSi₃O₈ adopts a structure essentially different from reedmergneritc NaBSi₃O₈, with the low albite (NaAlSi₃O₈) structure, and isotypic with danburite CaB₂Si₂O₈ which has the same topology as paracelsian BaAl₂Si₂O₈. The chemical relationship between this sample and danburite gives insight into a new coupled substitution; K⁺ + Si⁴⁺ = Ca²⁺ + B³⁺ in the extraframework and tetrahedral sites. The present occupancy refinement revealed partial disordering of B and Si atoms which jointly reside in two kinds of general equivalent points, T(1) and T(2) sites. Thus the expanded crystal-chemical formula can be written in the form K(B_0.44Si_0.56)₂(B_0.06Si_0.94)₂O₈.

The systematic trend among crystalline compounds with the M⁺T³⁺T⁴⁺₃O₈ formula suggests that they exist in one of four structural types; the feldspar structures with T³⁺/T⁴⁺ ordered and/or disordered forms, and the paracelsian and the hollandite structures.