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Crystal-Chemistry and Short-Range Order of Fluoro-Edenite and Fluoro-Pargasite: a Combined X-Ray Diffraction and FTIR Spectroscopic Approach

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Crystal-Chemistry and Short-Range Order of Fluoro-Edenite and Fluoro-Pargasite: a Combined X-Ray Diffraction and FTIR Spectroscopic Approach Mineralogical Magazine, 2014, Vol. 78(2), pp. 293–310 Crystal-chemistry and short-range order of fluoro-edenite and fluoro-pargasite: a combined X-ray diffraction and FTIR spectroscopic approach 1,2, 1,2 3,4 5 G. DELLA VENTURA *, F. BELLATRECCIA ,F.CA´ MARA AND R. OBERTI 1 Dipartimento di Scienze, Universita` di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Rome, Italy 2 INFN, Laboratori Nazionali di Frascati, Via E. Fermi, 40, 00044 Frascati, Rome, Italy 3 Dipartimento di Scienze della Terra, via Valperga Caluso 35, I-10125 Turin, Italy 4 CrisDi, Interdepartmental Centre for Crystallography, Via P. Giuria 5, 10125, Turin, Italy 5 CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, I-27100 Pavia, Italy [Received 12 December 2013; Accepted 15 December 2013; Associate Editor: S.J. Mills] 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 0.00À0.05 a.p.f.u., atoms per formula unit) and Na is the dominant A cation, with A& (vacancy) in the range 0.07À0.21 a.p.f.u.; WF is in the range 1.18À1.46 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 cmÀ1, which are assigned to the amphibole and the systematic presence of two very broad absorptions at 3560 and 3430 cmÀ1.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 <MÀO> bond distance analysis. The measured frequencies of all components are typical of OÀH groups pointing towards SiÀO(7)ÀAl 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 SiÀO(7)ÀSi linkages regularly alternate with SiÀO(7)ÀAl linkages along the double chain. KEYWORDS: fluoro-edenite, fluoro-pargasite, EMPA, single-crystal structure refinement, FTIR powder and single-crystal spectra, FPA imaging, short-range order. This paper is published as part of a special issue of * E-mail: [email protected] Mineralogical Magazine, Volume 78(2), 2014, in DOI: 10.1180/minmag.2014.078.2.05 celebration of the International Year of Crystallography. # 2014 The Mineralogical Society G. DELLA VENTURA ET AL. Introduction gave: Na0.99(Ca1.84Na0.16)(Mg4.79Al0.18) (Si7.12Al0.88)O22F2.15. Graham and Navrotsky EDENITE, ideally NaCa2Mg5(Si7Al)O22(OH)2,is (1986) obtained high-yield amphibole run an endmember of the Ca amphibole subgroup products, with <5% pyroxene impurities along (Hawthorne et al., 2012) and hence an hydroxyl- the fluoro-tremoliteÀfluoro-edenite join. bearing double-chain silicate. It was first Raudsepp et al. (1991) also failed to synthesize described by A. Breithaupt in 1874 based on a edenite in the OH-system, but obtained high specimen from Franklin Furnace, New Jersey, fluoro-edenite yields (>90%) at P = 1 atm; in their USA (Clark, 1993). However, neither that sample case, EMPA showed significant deviation towards nor the one studied later by Palache (1935) from tschermakite. Boschmann et al. (1994) synthe- the Franklin marble falls within the compositional sized fluoro-edenite (with BMg contents close to range of edenite, as defined by the succeeding 0.20 a.p.f.u.) using the same technique (slow International Mineralogical Association cooling from 1240ºC) and characterized their Subcommittees on Amphiboles, i.e. then BCa 5 product by X-ray single-crystal diffraction and 1.50 a.p.f.u. (atoms per formula unit), A(Na,K) 5 EMPA. The crystals were strongly zoned, from a 0.50 a.p.f.u., 7.50 < TSi < 6.50 a.p.f.u. and core with composition close to endmember presently W(OH,F) and ANa dominant, fluoro-edenite to a rim with composition inter- BCa/B(Ca+Na) 5 0.75, 0.00< CR3+ 40.50 (e.g. mediate between fluoro-tremolite and fluoro- Hawthorne et al., 2012). In fact, re-examination pargasite. X-ray single-crystal refinement of several samples from the marble quarries in the (SREF) showed that TAl is strongly ordered at Franklin area, kept and labelled as edenite in T(1) and suggested that CAl is strongly ordered at various Mineralogical Museums all around the M(2), the latter being a conclusion that should be world, has shown these to be pargasite, or edenitic revised in the light of this work. Gianfagna and magniesio-hornblende or pargasitic-hornblende Oberti (2001) reported on the occurrence of (our unpublished work; M. Lupulescu, pers. fluoro-edenite very close to the ideal composition, comm.) NaCa2Mg5Si7AlO22F2, within the cavities of an Several attempts have been made to grow altered benmoreitic lava at Biancavilla (Catania, edenite experimentally. However, in OH-bearing Italy). Oberti et al. (1997) characterized a systems the synthesis of endmember edenite has synthetic Mn-rich fluoro-edenite never been successful (Gilbert et al., 1982). Na et [Na1.06(Ca1.41Na0.12Mn0.47)(Mg4.44Mn0.41Al0.15) 2+ al. (1986) investigated the phase relations in the (Si6.91Al1.09)O22F2.00] and commented on Mn system edenite + H2O and edenite + excess quartz partitioning. Later, Oberti et al. (2006) reported A +H2O. They found that, when quartz is not in on the occurrence of (Na0.74K 0.02) B C 2+ 2+ 3+ excess, phlogopite is the stable phase over a wide (Ca1.27Mn0.73) (Mg4.51Mn0.28Fe0.05Fe0.03Al0.12 T range of conditions including the amphibole P-T Ti0.01) (Si7.07Al0.93)O22(OH)2 – at that time the field, thus suggesting that edenite is probably new endmember parvo-mangano-edenite but now B stable only under very high aSiO2. Indeed, excess simply Mn-rich edenite after Hawthorne et al. quartz in the system stabilizes the amphibole; (2012) – in the Grenville Marble of the Arnold however, the compositions obtained are system- mine, Fowler, St. Lawrence Co., New York atically solid-solutions in the ternary system (USA). They examined the possible bond- tremolite–richteriteÀedenite. Della Ventura and valence configurations proposed for edenite by Robert (unpublished data) failed to synthesize Hawthorne (1997) and showed that the presence B B edenite at T = 700ºC and PH2O = 1 kbar. Della of Mn substituting for Ca helps to stabilize the Ventura et al. (1999) studied the richterite– charge arrangement of edenite. pargasite join and observed that the amphibole In summary, the rarity of natural amphiboles is substituted by a Na-rich mica when with the edenite composition, their constant approaching the edenite stoichiometry. In enrichment in Fe or Mn and the failure in contrast, fluoro-edenite has been obtained easily obtaining synthetic analogues of edenite, suggest by several authors in OH-free systems. Kohn and that this amphibole is probably not stable Comeforo (1955) synthesized fluoro-edenite and (Raudsepp et al., 1991) as confirmed by bond- B-rich fluoro-edenite starting from a mixture of valence considerations (Hawthorne, 1997). oxides, at P = 1 atm and using the melting Systematic work was undertaken here aimed at technique, with slow cooling from 1350ºC. The obtaining new structural and crystal-chemical wet-chemical analysis of the bulk run-product data on edenite and fluoro-edenite; this study is 294 SHORT-RANGE ORDER IN FLUORO-AMPHIBOLES focused on selected fluoro-edenite and fluoro- c/o Universita` di Roma La Sapienza. Analytical pargasite samples from the Franklin marble conditions were 15 kV accelerating voltage and obtained from various mineral collections. A 15 nA beam current, with a 5 mm beam diameter. multi-methodological approach was used, which Counting time was 20 s on both peak and includes EMPA, single-crystal X-ray diffraction background. The standards used were: wollasto- and refinement and FTIR spectroscopy. In the last nite (SiKa, TAP; Ca Ka, PET), periclase (MgKa, two decades, FTIR spectroscopy in the OH- TAP), corundum (AlKa TAP), orthoclase (KKa, stretching region has been used extensively in PET), jadeite (NaKa, TAP), magnetite (FeKa, the study of hydrous minerals (e.g. Libowitzky LIF), rutile (TiKa, LIF), Mn, Zn and Cr metal and Beran, 2004 and references therein) and (MnKa,ZnKa, LIF; CrKa PET), synthetic fluoro- proved to be a fundamental tool for characterizing phlogopite (FKa, TAP) and sylvite (ClKa, PET). short-range order/disorder features in amphiboles Analytical errors are 1% rel. Data reduction was (Hawthorne and Della Ventura, 2007). In this performed with the PAP method (Pouchou and paper, FTIR spectroscopy also allowed us to Pichoir, 1985). address two poorly explored issues in amphibole The crystal-chemical formulae were calculated crystal-chemistry, namely the SRO of Mg/Al at based on 24 (O,OH,F,Cl) and 2 (OH,F,Cl) a.p.f.u., the M(1À3) sites in the ribbon of octahedra and of as suggested by the structure refinement which Si/Al at the T(1,2) sites in the double chain of excluded the presence of a significant oxo tetrahedra.
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