Crystal-chemistry and short-range order of fluoro-edenite and fluoro-pargasite : a combined X-ray diffraction and FTIR spectroscopic approach

This study addresses the crystal chemistry of a set of five samples of F-rich amphiboles from the Franklin marble (USA), using a combination of microchemical (Electron microprobe analysis (EMPA)), single-crystal refinement (SREF) and Fourier transform infrared (FTIR) spectroscopy methods. The EMPA data show that three samples fall into the compositional field of fluoro-edenite (Hawthorne et al., 2012), whereas two samples are enriched in high-charged C cations and - although very close to the CR3+ boundary - must be classified as fluoro-pargasite. Magnesium is by far the dominant C cation, Ca is the dominant B cation (with BNa in the range 0.00-0.05 a.p.f.u., atoms per formula unit) and Na is the dominant A cation, with A\u25a1 (vacancy) in the range 0.07-0.21 a.p.f.u.; WF is in the range 1.18-1.46 a.p.f.u. SREF data show that: TAl is completely ordered at the T(1) site; the M(1) site is occupied only by divalent cations (Mg and Fe2+); CAl is disordered between the M(2) and M(3) sites; ANa is ordered at the A(m) site, as expected in F-rich compositions. The FTIR spectra show a triplet of intense and sharp components at 3c3690, 3675 and 3660 cm-1, which are assigned to the amphibole and the systematic presence of two very broad absorptions at 3560 and 3430 cm-1. These latter are assigned, on the basis of polarized measurements and FPA (focal plane array) imaging, to chlorite-type inclusions within the amphibole matrix. Up to eight components can be fitted to the spectra; band assignment based on previous literature on similar compositions shows that CAl is disordered over the M(2) and M(3) sites, thus supporting the SREF conclusions based on the bond distance analysis. The measured frequencies of all components are typical of O-H groups pointing towards Si-O(7)-Al tetrahedral linkages, thus allowing characterization of the SRO (shortrange- order) of TAl in the double chain. Accordingly, the spectra show that in the fluoro-edenite/ pargasite structure, the T cations, Si and Al, are ordered in such a way that Si-O(7)-Si linkages regularly alternate with Si-O(7)-Al linkages along the double chain