Fluoronyboite, mineral description and relations with other high-pressure amphibole end-members. (Abstract/Poster in atti di convegno)

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  • Fluoronyboite, mineral description and relations with other high-pressure amphibole end-members. (Abstract/Poster in atti di convegno) (literal)
Anno
  • 2003-01-01T00:00:00+01:00 (literal)
Alternative label
  • R. Oberti1, M. Boiocchi2,3, D.C. Smith4 (2003)
    Fluoronyboite, mineral description and relations with other high-pressure amphibole end-members.
    in GEOITALIA 2003. Forum Italiano di Scienze della Terra., Bellaria, 16-18 settembre 2003
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • R. Oberti1, M. Boiocchi2,3, D.C. Smith4 (literal)
Pagina inizio
  • 645 (literal)
Pagina fine
  • 646 (literal)
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  • GEOITALIA 2003, Abstracts (literal)
Note
  • Abstract (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • 1CNR-Istituto di Geoscienze e Georisorse, sezione di Pavia 2Centro Grandi Strumenti, Università degli Studi di Pavia 3Dipartimento di Scienze della Terra, Università degli Studi di Pavia 4Muséum National d'Histoire Naturelle, Bâtiment de Minéralogie, Paris (literal)
Titolo
  • Fluoronyboite, mineral description and relations with other high-pressure amphibole end-members. (literal)
Abstract
  • The new amphibole end-member fluoronyböite, ideally Na Na2 (Al2Mg3) (Si7Al) O22 F2, has been found in the Jianchang eclogite pod, Su-Lu coesite-eclogite province, China, described by Smith et al. (1990) and Yang (1991). Fluoronyböite formed during UHPM conditions, and is preserved in the retrograded kyanite-bearing eclogite sample DJ102 together with clinopyroxene (Jd70Ae20Di10), garnet (Alm60Prp21Grs17Sps02), and rutile. Lower-pressure minerals are also present [fluoro alumino-magnesiotaramite, apatite, paragonite], as well as the development of plagioclase-bearing symplectitic rims around clinopyroxene crystals. Unit-cell parameters of fluoronyböite are: a = 9.666(4), b = 17.799(6), c = 5.311(2) Å, ? = 104.10(3)º, V = 886.2(8) Å3; the unit formula is: A(Na0.78 K0.06)?0.84 B(Na1.53 Ca0.47)?2.00 C(Fe2+0.89 Mg2.55 Mn0.01 Zn0.01 Fe3+0.32 Al1.21 Ti0.01)?5.00 T(Si7.14 Al0.86)?8.00 O22 X(OH0.84 F1.16)?2.00. Cation ordering, structural and physical properties of fluoronyböite will be discussed with reference to those of F-free nyböite from the type locality at Nybö in Norway (Ungaretti et al., 1981). Comparative crystal-chemical analysis shows that in nyböitic amphiboles, as well as in other high-pressure end-members (e.g. glaucophane), the M1 and M3 octahedra are larger than expected on the basis of their site populations. In fluoronyböite, the observed contraction of the M1 and M3 sites due to the F(OH)-1 exchange is far smaller than expected on the basis of the behaviour of the other end-members. Geometrical requirements for the matching of the different structural components implies relaxation of the strip of octahedra, and/or incorporation of octahedral cations larger than Mg. Their site preference is M3>M1>M2. The occurrence of stoichiometric end-member compositions is therefore unlikely for nyböites, as already observed for glaucophanes. Fluoronyböite and nyböite occur in eclogites which share a number of UHPM characteristics (Nybö, Liset, Jianchang) and nowhere else to date (except Brossasco-Isasca for Fe2+-richer compositions); it is possible that they are stable in Nature in the coesite-eclogite subfacies (i.e. up to at least 32 kbar at 800 °C); however, since these rocks evolved along a depressurisation path through the quartz-eclogite facies to the amphibolite facies, it is also possible that they may have grown only in the quartz-eclogite-facies (i.e. around 23 kbar at 750 °C). They are certainly pre-amphibolite-facies minerals because alumino-magnesiotaramite replaces them in the amphibolite-facies at the two type localities, Nybö and Jianchang, and also at Liset, Norway (and also in Italy for the Fe2+-rich analogues). Smith, D.C., Yang, J., Oberti, R. and Previde-Massara, E. 15th IMA Assembly, Abstracts, 2, 889-890. (1990) Ungaretti, L., Rossi, G., and Smith, D.C. Bull. Soc. Franc. Miner. Crist., 104, 400-412 (1981). Yang, J. Geol. Soc. China. Geological Publishing House: 99 p. (1991) (literal)
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