Hostname: page-component-cd9895bd7-hc48f Total loading time: 0 Render date: 2024-12-24T00:14:31.652Z Has data issue: false hasContentIssue false
  • English
  • Français

Nb-rich brookite from Gross Brukkaros, Namibia: substitution mechanisms and Fe2+/Fe3+ ratios

Published online by Cambridge University Press:  05 July 2018

M. Werner  and
N. J. Cook
M. Werner*
Affiliation:
Institut für Geologie der Universität Würzburg, D-97070 Würzburg, Germany
N. J. Cook
Affiliation:
Geological Survey of Norway, N-7491 Trondheim, Norway
*
*E-mail: [email protected]
Get access Rights & Permissions [Opens in a new window]

Abstract

Idioblastic, mm-scale brookite is reported from SiO2-Ba-Fe-rich metasomatic rocks from the carbonatitic Gross Brukkaros Volcanic Field, Southern Namibia. Electron probe microanalysis shows significant concentrations of Nb (12–17 wt.% Nb2O5), coupled with Fe, substituting for Ti in the structure. Calculation of the analytical data indicates evolving Fe2+/(Fe2++Fe3+) ratios during crystallization and two distinct substitution mechanisms in operation: 2Ti4+ ⇋ Nb5+ + Fe3+ and 3Ti4+ ⇋ 2Nb5+ + Fe2+. The compositional patterns observed are explained in terms of initial growth in an environment which evolved from oxidizing to reducing. The outermost 10 µm rims of the crystals are characterized by oxidizing conditions, resulting from a subsequent metasomatic overprint.

Keywords

brookiteniobiumsubstitution mechanismscarbonatiteGross BrukkarosNamibia
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 3 , June 2001 , pp. 437 - 440
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Brito de Carvalho, L.H., Sahama, T.G. and Lopes Nunes, J.E. (1974) Niobian brookite from Mount Negoza-Carinde; Mozambique. Revista de Ciências Geológicas (Universidade de Lourenço Marques, Mozambique), Série A, 7, 61–8.Google Scholar
Černý, P., Chapman, R., Göd, R., Niedermayr, G. and Wise, M.A. (1989) Exsolution intergrowths of titanian ferrocolumbite and niobian rutile from the Weinebene spodumene pegmatites, Carinthia, Austria. Mineral. Petrol., 40, 197–206.CrossRefGoogle Scholar
Černý, P., Chapman, R., Simmons, W.B. and Chackowsky, L.E. (1999) Niobian rutile from the McGuire granitic pegmatite, Park County, Colorado: Solid solution, exsolution, and oxidation. Amer. Mineral., 84, 754–63.CrossRefGoogle Scholar
Flohr, M.J.K. (1994) Titanium, vanadium, and niobium mineralization and alkali metasomatism from the Magnet Cove Complex, Arkansas. Econ. Geol., 89, 105–30.CrossRefGoogle Scholar
Foord, E.E., Chirnside, W., Davis, A.M., Lichte, F.E. and Esposito, K.J. (1995) A new U-Ti-Ca-REE hydrated oxide and associated niobian rutile from Topaz Valley, Utah. Mineral. Record, 26, 123–8.Google Scholar
Gaspar, J.C. and Wyllie, P.J. (1983) Ilmenite (high Mg, Mn, Nb) in the carbonatites from the Jacupiranga Complex, Brazil. Amer. Mineral., 68, 960–71.Google Scholar
Howard, J.M. (1999) Brookite, rutile paramorphs after brookite, and rutile twins from Magnet Cove, Arkansas. Rocks and Minerals, 74, 92–102.CrossRefGoogle Scholar
Janse, A.J.A. (1969) Gross Brukkaros, a probable carbonatite volcano in the Nama Plateau of South-West Africa. Geol. Soc. Amer. Bull., 80, 573–86.CrossRefGoogle Scholar
Kurszlaukis, S. (1994) Geology and geochemistry of the carbonatitic Gross Brukkaros volcanic field and the ultrabasic Blue Hills Intrusive complex, southern Namibia.Unpubl. Thesis, Univ. Würzburg, Germany.Google Scholar
Kurszlaukis, S. and Lorenz, V. (1997) Volcanological features of a low-viscosity melt: the carbonatitic Gross Brukkaros Volcanic Field, Namibia. Bull. Volcanol., 58, 421–31.CrossRefGoogle Scholar
Nakashima, K. and Imaoka, T. (1998) Niobian and zincian ilmenites in syenites from Cape Ashizuri, Southwest Japan. Mineral. Petrol., 63, 1–17.CrossRefGoogle Scholar
Pearce, N.J.G. (1990) Zirconium and niobium-bearing ilmenites from the Igaliko dyke swarm, South Greenland. Mineral. Mag., 54, 585–8.CrossRefGoogle Scholar
Russell, J.K., Groat, L.A. and Halleran, A.A.D. (1994) LREE-rich niobian titanite from Mount Bisson, British Columbia; chemistry and exchange mechanisms. Canad. Mineral., 32, 575–87.Google Scholar
Shuriga, T.N., Gaydukova, V.S. and Ryabeva, Ye.G. (1987) Micro-heterogeneity of niobium-containing brookite from alkalic rare-metal metasomatic rocks of Primorye. Novye Dannye o Mineralah SSSR, 34, 157–62 (in Russian).Google Scholar
Stachel, T., Brey, G. and Lorenz, V. (1995) Carbonatite magmatism and fenitization of the epiclastic caldera- fill at Gross Brukkaros (Namibia). Bull. Volcanol., 57, 185–96.CrossRefGoogle Scholar
Verwoerd, W.J., Viljoen, E.A. and Chevallier, L. (1995) Rare metal mineralizat ion at the Salpeterkop carbonatite complex, Western Cape Province, South Africa. J. Afr. Earth Sci., 21, 171–86.CrossRefGoogle Scholar
Werner, M. (1998) Metasomatisch-hydrothermale Mineralisationen und Gesteinsüberprägungen im Bereich des karbonatitischen Gross Brukkaros-Vulkanfeldes im Süden Namibias. Unpubl. Thesis, Univ. Würzburg, Germany.Google Scholar
Woolley, A.R., Platt, R.G. and Eby, N. (1992) Niobian titanite and eudialyte from the Ilomba nepheline syenite complex, northern Malawi. Mineral. Mag., 56, 428–80.CrossRefGoogle Scholar

A correction has been issued for this article:

Errata