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

In this study we address the crystal-chemistry of a set of five samples of F-rich amphiboles from the Franklin marble (USA), using a combination of microchemical (EMPA), SREF, and 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. Mg is by far the dominant C cation, Ca is the dominant B cation (with BNa in the range 0.00-0.05 apfu, atoms per formula unit), and Na is the dominant A cation, with A (vacancy) in the range 0.07-0.21 apfu; WF is in the range 1.18-1.46 apfu. 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 ~ 3690, 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 <M-O> bond distance analysis. The measured frequencies of all components are typical of O-H groups pointing toward Si-O(7)-Al tetrahedral linkages, thus allowing to characterize the SRO (short-range-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.