Polarized single‐crystal Raman spectroscopy of danburite, CaB₂Si₂O₈

Polarized single‐crystal Raman spectra were collected from the borosilicate feldspar mineral danburite (CaB 2 Si 2 O 8 ) over the region 1800–40 cm −1 . The symmetry of danburite (orthorhombic) is higher than that of the other feldspars (monoclinic or triclinic) and this simplifies the vibrational analysis. A very high proportion (74/78) of the bands due to Raman‐active fundamentals are observed, making this the first such study in which most of the bands for a feldspar mineral have been observed. Assignment of bands to stretching, bending and lattice modes is problematic because of the framework structure of the feldspar. The physically unrealistic disconnection of the danburite lattice into [Si 2 O 7 ] 6− , [B 2 O] 4+ and Ca 2+ ions leads to remarkably good predictions of the numbers of bands for each symmetry species in the wavenumber ranges appropriate for the Si(B)OSi(B) stretching (1300–825 cm −1 ), bending (825–390 cm −1 ) and lattice (390–90 cm −1 ) modes.