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Magnesio-ferri-fluoro-hornblende from Portoscuso, Sardinia, Italy: description of a newly approved member of the amphibole supergroup

Published online by Cambridge University Press:  02 January 2018

Roberta Oberti ,
Massimo Boiocchi ,
Frank C. Hawthorne ,
Neil A. Ball  and
Luigi Chiappino
Roberta Oberti*
Affiliation:
CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, I-27100 Pavia, Italy
Massimo Boiocchi
Affiliation:
Centro Grandi Strumenti, Università di Pavia, via Bassi 21, I-27100 Pavia, Italy
Frank C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, MB, R3 T 2N2, Canada
Neil A. Ball
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, MB, R3 T 2N2, Canada
Luigi Chiappino
Affiliation:
Via Palmanova 67, I-20132 Milano, Italy
*
*E-mail: oberti@crystal.unipv.it
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Abstract

Magnesio-ferri-fluoro-hornblende has the ideal formula A□B Ca2C(Mg4Fe3+)T(Si7Al)O22WF2 (Hawthorne et al., 2012). The holotype sample described in this work occurs as prismatic crystals in vugs of volcanic rocks (Seruci ignimbrites), found along the coast road ∼5.5 km northeast of Portoscuso, Cagliari, Sardinia; associated minerals are tridymite, todorokite, magnetite, and hematite. The name and the mineral were approved by the IMA CNMNC (2014-091). Holotype magnesio-ferri-fluoro-hornblende is monoclinic, space group C2/m, a = 9.839(5), b = 18.078(9), c = 5.319(3) Å, β = 104.99(3)°, V = 913.9(9) Å3, Z = 2. The density calculated from the empirical formula is 3.315 g cm–3. In plane-polarized light, magnesio-ferri-fluoro-hornblende is pleochroic, X = pale grey (least), Y = dark grey (most), Z = pale brownish grey (intermediate); X^ a = 47.6° (β obtuse), Y // b, Z^ c = 33.4° (β acute). It is biaxial negative, α = 1.669, β = 1.676, γ = 1.678, all ±0.002; 2Vobs = 74(1)°, 2Vcalc = 56°. The strongest eight lines in the powder X-ray diffraction pattern are [d in Å (I)(hkl)]: 2.711 (100)(151), 8.412 (89)(110), 3.121 (64)(310), 2.553 (61)(2̄02), 3.389 (55)(131), 2.599 (45)(061), 2.164 (36)(261), and 2.738 (34)(3̄31). Electron-microprobe analysis of the refined crystal gave SiO2 45.34, Al2O3 6.18, TiO2 1.22, FeO 15.24, Fe2O3 6.27, MgO 9.71, MnO 0.78, ZnO 0.06, CaO 10.18, Na2O 1.35, K2O 1.15, F 3.22, Cl 0.30, H2Ocalc 0.37, sum 99.95 wt.%. The empirical formula unit, calculated on the basis of 24 (O, OH, F, Cl) apfu with (OH + F + Cl) = 2 apfu is: (Na0.15K0.22)∑0.37(Na0.25Ca1.66Mn0.09)∑2.00(Mg2.20Fe2+1.94Mn0.01Zn0.01Fe3+0.72Ti0.13)∑5.01 (Al1.11Si6.89)∑8.00O22[F1.55(OH)0.37Cl0.08)∑2.00.

Keywords

Magnesio-Ferri-Fluoro-HornblendeAmphiboleElectron-Microprobe AnalysisCrystal-Structure RefinementSardinia
Type
Research Article
Information
Mineralogical Magazine , Volume 80 , Issue 2 , April 2016 , pp. 269 - 275
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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References

Assorgia, A., Fadda, A., Gimeno, D., Morra, V., Ottelli, L. and Secchi, F.A. (1990) Le successioni ignimbritiche terziarie del Sulcis (Sardegna sud-occidentale). Memorie Società Geologica Italiana, 45, 951—963 Google Scholar
Assorgia, A., Brotzu, P., Callegari, E., Fadda, A., Lonis, R., Ottelli, L., Ruffini, R. and Abrate, T. (1992) Carta geologica del distretto vulcanico cenozoico del Sulcis (Sardegna sud-occidentale). S.EL.CA., Florence, Italy.Google Scholar
Bartelmehs, K.L., Bloss, F.D., Downs, R.T. and Birch, J.B. (1992) EXCALIBR II. Zeitschrift für Kristallographie, 199, 185–196. CrossRefGoogle Scholar
Bruker, (2003) SAINT Software Reference Manual. Version 6. Bruker AXS Inc., Madison, Wisconsin, USA.Google Scholar
Czamanske, G.K. and Wones, D.R. (1973): Oxidation during magmatic differentiation, Finnmarka Complex, Oslo area, Norway. 2. The mafic silicates. Journal of Petrology, 14, 349–380. CrossRefGoogle Scholar
Hawthorne, F.C. and Oberti, R. (2007) Amphiboles: crystal chemistry. Pp. 1-54 in: Amphiboles: Crystal Chemistry, Occurrence and Health Issue. (F.C. Hawthorne and R. Oberti, editors). Reviews in Mineralogy and Geochemistry, 67. Mineralogical Society of America, Washington D.C., and the Geochemical Society, St. Louis, Missouri, USA.CrossRefGoogle Scholar
Hawthorne, F.C., Ungaretti, L. and Oberti, R. (1995) Site populations in minerals: terminology and presentation of results of crystal-structure refinement. The Canadian Mineralogist, 33, 907—911 Google Scholar
Hawthorne, F.C., Oberti, R. and Sardone, N. (1996) Sodium at the A site in clinoamphiboles: the effects of composition on patterns of order. The Canadian Mineralogist, 34, 577–593. Google Scholar
Hawthorne, F.C., Oberti, R., Harlow, G.E., Maresch, W.V., Martin, R.F., Schumacher, J.C. and Welch, M.D. (2012) Nomenclature of the amphibole supergroup. American Mineralogist, 97, 2031—2048 CrossRefGoogle Scholar
Morra, V., Secchi, F.A. and Assorgia, A. (1994) Petrogenetic significance of peralkaline rocks from Caenozoic calc-alkaline volcanism from SW Sardinia, Italy. Chemical Geology, 118, 109–142.Google Scholar
Oberti, R., Ungaretti, L., Cannillo, E., Hawthorne, F.C. (1992) The behaviour of Ti in amphiboles: I. Four- and six-coordinated Ti in richterites. European. Journal of Mineralogy, 4, 425–439.CrossRefGoogle Scholar
Oberti, R., Ungaretti, L., Cannillo, E., Hawthorne, F.C. and Memmi, I. (1995) Temperature-dependent Al order-disorder in the tetrahedral double-chain of C2/m amphiboles. European Journal of Mineralogy, 7, 1049–1063.CrossRefGoogle Scholar
Oberti, R., Hawthorne, F.C., Cannillo, E. and Cámara, F. (2007) Long-range order in amphiboles. Pp. 125–171 in: Amphiboles: Crystal Chemistry, Occurrence and Health Issues (F.C. Hawthorne, R. Oberti, G. Della Ventura and A. Mottana, editors). Reviews in Mineralogy and Geochemistry, 67. Mineralogical Society of America, Washington D.C., and the Geochemical Society, St. Louis, Missouri, USA.CrossRefGoogle Scholar
Ottelli, L. and Perna, G. (1993) Carta geologica del bacino carbonifero del Sulcis (Sardegna sud occidentale). S.EL.CA., Florence, Italy.Google Scholar
Pouchou, J.L. and Pichoir, F. (1985) ‘PAP’ j(ρZ) procedure for improved quantitative microanalysis. Pp. 104–160 in: Microbeam Analysis (J.T. Armstrong, editor). San Francisco Press, San Francisco, USA.Google Scholar
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