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Experimental Study of Cr Incorporation in Pargasite Claire-Isabelle Fialips-Guédon, Jean-Louis Robert, François Delbove

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Experimental Study of Cr Incorporation in Pargasite Claire-Isabelle Fialips-Guédon, Jean-Louis Robert, François Delbove Experimental study of Cr incorporation in pargasite Claire-Isabelle Fialips-Guédon, Jean-Louis Robert, François Delbove To cite this version: Claire-Isabelle Fialips-Guédon, Jean-Louis Robert, François Delbove. Experimental study of Cr incor- poration in pargasite. American Mineralogist, Mineralogical Society of America, 2000, 85, pp.687-693. hal-00077529 HAL Id: hal-00077529 https://hal-insu.archives-ouvertes.fr/hal-00077529 Submitted on 31 May 2006 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. American Mineralogist, Volume 85, pages 687–693, 2000 Experimental study of Cr incorporation in pargasite CLAIRE-ISABELLE FIALIPS-GUÉDON,1 JEAN-LOUIS ROBERT,2 AND FRANÇOIS DELBOVE2 1Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, California 95616, U.S.A. 2Centre de Recherche sur la Synthèse et la Chimie des Minéraux, and FR 09, CNRS, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France ABSTRACT Pargasite compositions corresponding to the nominal structural formula NaCa2 ≤ ≤ ° (Mg4Al1-xCrx)(Si6Al2)O22(OH)2, with 0 x 1, were investigated at 900 C and 3 and 20 kbar PH2O. The aims of this work were to determine the extent of the solid solution, i.e., the maximum x value, as a function of experimental conditions, and to characterize the M3+ (Al3+ and/or Cr3+) distribution over the M sites. A first series of experiments at 900 °C and 3 kbar showed the presence of large 6+ amounts of Cr as sodium chromate (Na2CrO4) and dichromate (Na2Cr2O7) in the final fluid phase, which promotes the extraction of Cr from the bulk solid (up to 45 mol%). The oxidizing agent was the oxygen from water, liberated by diffusion of hydrogen from water through the Pt tube wall. To prevent this oxidation, a new double-chamber tube was designed, one chamber containing powdered metallic Cr to trap the excess of oxygen and the second chamber initially containing the starting gel and water. Using this tube, (chromium)-pargasites were produced with a maximum Cr content around 0.43 apfu from EMP analyses (based on 23 O atoms), close to the maximum Cr solubility observed in naturally occurring pargasites. In FTIR spectra of Cr-free pargasite, two intense OH-stretching –1 bands are observed at 3714 and 3681 cm , corresponding to OH groups adjacent to Mg3 and to –1 Mg2Al, respectively, and pointing toward a filled A-site. A third band is observed at 3656 cm , which can be assigned to OH groups pointing toward vacant A-sites. Along the Al-Cr pargasite join, sig- nificant modifications are observed in the OH-stretching region: a new band appears at 3660 cm–1, which can be assigned to OH groups adjacent to Mg2Cr, and pointing toward a filled A-site. Its relative intensity increases regularly with the Cr content, showing that Cr3+ enters M site(s) adjacent to an OH group. Combining EMP and FTIR data, we conclude that up to 0.27 Cr per formula unit occupies the M3 site. However, these are more octahedral trivalent cations than expected, and these are balanced by lower tetrahedral Al and lower occupancies of the A and M4 sites. This charge arrangement, with important deviations from ideal stoichiometry, is apparently the only stable one ° under the conditions applied. Results were confirmed at 900 C and 20 kbar PH2O, indicating that increasing pressure does not affect Cr solubility in pargasite. INTRODUCTION cesses affecting the upper mantle (Veblen and Ribbe 1982). Pargasite is a monoclinic amphibole end-member with ideal Chromium-rich pargasites commonly occur as inclusions in ophiolitic complexes and in chromitites from mafic-ultramafic formula NaCa2Mg4AlSi6Al2O22(OH)2. The tetrahedral double- chain is composed of T1 and T2 sites. T1 sites accommodate layered complexes (Johan et al. 1983). The maximum reported Si and Al whereas T2 sites accommodate mainly Si. The octa- chromium content of pargasite seems to be around 3.5 wt% of hedral strip is composed of two M1, two M2, and one M3 site Cr2O3, i.e., close to 0.5 Cr atom per formula unit (apfu) (Johan per formula unit. The M1 and M3 sites are adjacent to four O et al. 1983). For example, the average structural formula, based atoms and two hydroxyl groups, in cis and trans positions, re- on several tens of analyses of (chromium)-pargasite inclu- spectively, whereas the M2 site is adjacent to six O atoms. In sions in chromites from the Oman ophiolitic complex, is: 2+ the present study, these octahedral sites can accommodate Mg2+, (Na0.57K 0.03)(Ca1.57Mg0.43)(Mg3.91Fe 0.30Cr0.42Al0.22Ti0.17) Al3+, and Cr3+. The M4 site is a wide, anti-prismatic pseudo- (Si6.38Al1.62)O22(OH)2 (Augé, personal communication, 1996). cubic cavity that can accommodate mainly Na+ and Ca2+ but These amphiboles, included in chromites, are commonly asso- also Mg2+. In pargasite, the A-site is mostly occupied by Na+, ciated with phases of the high-temperature decomposition but it can be also partly vacant. paragenesis of pargasite: Al-rich diopside, forsterite, nepheline, Pargasites are encountered from the Earth’s upper mantle spinel (the chromite itself), anorthite, plus Na-rich trioctahedral to the crust (Deer et al. 1992). Their origin in mantle rocks is micas (Johan et al. 1983). controversial: some authors consider pargasites to be primary Experiments were performed under hydrothermal phases, stable at the liquidus of basic melts, whereas other ones conditions along the NaCa2(Mg4Al)(Si6Al2)O22(OH)2- ° consider these amphiboles to be the result of metasomatic pro- NaCa2(Mg4Cr)(Si6Al2)O22(OH)2 join, at 900 C and 3 and 20 kbar PH2O, using different fluid compositions. The aims of the study were to analyze the Cr solubility in pargasites, and to *E-mail: [email protected] understand the incorporation mechanism of Cr into the am- 0003-004X/00/0506–687$05.00 687 688 FIALIPS-GUÉDON: CHROMIUM INCORPORATION IN PARGASITE phibole structure and its distribution between the different sites. time of 5 s per step. Unit-cell parameters (a, b, c, and β) of The crystal-chemical approach is based on X-ray diffraction synthesized pargasites were refined with the “Least-Squares (XRD) data, and Fourier transform infrared absorption spec- Unit-Cell Refinement” Program of Appleman and Evans (1973), trometry (FTIR) measurements in the OH-stretching region. as modified by Garvey (1987). A set of at least 30 unambigu- The valence state of Cr in experimental systems was also con- ously indexed peaks was used for pargasites corresponding to sidered. low nominal XCr values (≤0.6). For higher bulk Cr contents, the increasing proportion of pyroxene reduced to about 20 the num- MATERIALS AND METHODS ber of unambiguous diffraction lines used for the refinement Six gels of different compositions were prepared accord- of the amphibole cell dimensions. ing to the gelling method of Hamilton and Henderson (1968), 0 Fourier transform infrared absorption spectrometry using high-grade Na2CO3, CaCO3, and Cr (transformed to 3+ –1 Na, Ca, and Cr nitrates by dissolution in HNO3), titrated. Infrared spectra were recorded in the 3800–3500 cm range, Mg and Al nitrate solutions, and tetraethylorthosilicate with a 2 cm–1 nominal resolution, on a Nicolet 710 spectrom- (TEOS) as starting reagents (Guédon 1996). Samples were eter swept by a permanent dry air flow. The scanning condition synthesized along the theoretical NaCa2(Mg4Al)(Si6Al2) was 400 scans for both the background and the samples. Spec- O22(OH)2-NaCa2(Mg4Cr)(Si6Al2)O22(OH)2 join, by replacing tra were obtained by transmission mode from KBr pressed pel- [6]Al with [6]Cr in the starting gel, with nominal compositions lets. Pellets were prepared by homogeneous mixing 7–8 mg of [6] [6] measured by the atomic fraction of Cr [0 ≤ XCr = Cr / (Cr + dry sample with 150 mg of KBr. Al) ≤ 1]. Infrared spectra were decomposed in symmetrical Gaussians Two series of experiments were performed at 900 °C and 3 with the SPC program (Roux and Volfinger 1996); the reason kbar, for 3 days in an internally heated pressure vessel, using of the use of symmetrical Gaussians was discussed previously Ar as the pressure medium. These conditions were chosen ac- by Strens (1974) and Papin et al. (1997). cording to previous experimental studies on pargasite (Della Quantitative analysis of the cation site populations adja- Ventura et al. 1999). cent to OH groups from FTIR data needs the knowledge of For the first series of syntheses, 160–190 mg of starting gel molar extinction coefficients. Absolute values of these extinc- and 24–29 µL of solution (15 wt%) were introduced into Pt tion coefficients are not available for most cationic environ- tubes. The solution used was distilled water for the most part, ments; however, we know that they remain constant when the but three experiments were performed using increasingly con- wavenumber changes, even when transition elements are in- centrated NaCl solutions (1–4 M). volved, provided that the second- and third-neighbor cationic For the second series of syntheses, a double-chamber tube environments remain unchanged in the amphibole structure was designed, one chamber initially containing 120–240 mg (Della Ventura et al. 1996, 1997). of starting gel and 18–36 µL of distilled water (15 wt%), and the second chamber containing 300 mg of powdered metallic Electron microprobe Cr.
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  • Crystal-Chemistry and Short-Range Order Of
    Short-range order in fluoro-amphiboles 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 GIANCARLO DELLA VENTURA , FABIO BELLATRECCIA , FERNANDO 3 4 CÁMARA AND ROBERTA OBERTI 1) Dipartimento di Scienze, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy 2) INFN, Laboratori Nazionali di Frascati, Roma 3) Dipartimento di Scienze della Terra, via Valperga Caluso 35, I-10125 Torino 4) CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, I-27100 Pavia, Italy 1 Short-range order in fluoro-amphiboles ABSTRACT 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.
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  • Serendibite from the Northwest Adirondack Lowlands, in Russell, New York, USA
    SHORT COMMUNICATIONS 133 (Me75-83), aluminous diopside, green amphibole, Acknowledgements. This work was supported by con- colourless spinel, large poikilitic blue tourmaline, tracts CNRS-INSU 89 DBT V-11 and DBT 96. and a little calcite. Tourmaline includes several minerals, especially clinopyroxene, spinel, and a References few small crystals of serendibite. Scattered grains of the latter in one crystal of tourmaline are in de Roever, W. F. and Kieft, C. (1976) Am. Mineral. optical continuity, suggesting that tourmaline is 61,332-3. Grew. E. S. (1983) Mineral. Mag. 47,401-3. a breakdown product after serendibite. The ser- -- (1988) Am. Mineral. 73,345-57. endibite is colourless to very light green. The Haslam, H. W. (1980) Mineral. Mag. 43,822-3. mineral is much less coloured than the prussian Huijsmans, J. P. P., Barton, M., and van Bergen, blue crystals in a calcsilicate rock associated with M. J. (1982) NeuesJahrb. Mineral. Abh. 143, 24%61. clintonite clinopyroxenites from Ianapera, in SW Hutcheon, I., Gunter, A. E. and Lecheminant, A. N. Madagascar (Nicollet, 1988, 1990). The Ihosy ser- (1977) Can. Mineral. 15,108-12. endibite has low birefringence and fine polysyn- Lee, S. M. and Holdaway, M. J. (1977) Geophys. thetic twinning. It may be mistaken for Monogr. 20, 7%94. sapphirine, but it is distinguishable from the latter Lonker, S. W. (1988) Contrib. Mineral. Petrol. 98, by a larger extinction angle and by its occurrence 502-16. M6gerlin, N. (1968) C.R. Sere. GOol. Madagascar, 67-9. in calcic rocks. The ferromagnesian minerals in Nicollet, C. (1985) PrOcambr. Res. 28,175-85.
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