Abstract

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□ (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 ~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 towards Si–O(7)–Al tetrahedral linkages, thus allowing characterization of 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.

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