Crystal structure of holtite I

The crystal structure of As-containing holtite I is refined (Ital Structures diffractometer, 939 crystallographically independent reflections, anisotropic approximation, R = 0.047). The parameters of the orthorhombic unit cell are a = 4.695(1) Å, b = 11.906(3) Å, c = 20.38(3) Å, sp. gr. Pnma, Z = 4. On the whole, the structural formula obtained, (Si2.43Sb0.36As0.21)BO3 [(Al0.62Ta0.26)Al2(Al0.98)2 (Al0.94)2O12](O,OH)2.65, corresponds to the electron-probe analysis data. The statistical replacement of (Si,As)O4 tetrahedra by pyramidal [SbO3] groups is confirmed. The X-ray diffraction spectra of holtite I are compared with those of holtite II