The crystal-chemistry of riebeckite, ideally Na2Fe23+ Fe-2(3)+ Si8O22(OH)(2) : a mufti-technique study

Fourier-Transform Infrared (FTIR) spectra. collected both on powders and single crystals, are presented and discussed. FTIR spectra in the NIR region are also presented for the first time for this amphibole. The FTER data are compatible with complete local ordering of A cations close to F, and complete Fe2+ /Mg disorder at M(1,3). Polarized Raman-scattering data collected from single crystals confirm this conclusion. In addition, it was found that FTIR data collected on powders provide the best agreement with the site occupancies derived from chemical (EMPA and LA-ICP-MS) and crystal-chemical data, possibly because they do not depend on experimental issues such as orientation and polarization.