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Nb-rich baotite in carbonatites and fenites at Haast River, New Zealand

Published online by Cambridge University Press:  05 July 2018

Alan F. Cooper
Alan F. Cooper*
Affiliation:
Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
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Abstract

Baotite occurs as an accessory mineral in carbonatites, fenites, and carbothermal veins associated with a lamprophyre dyke swarm in the Haast River area of south Westland, New Zealand. Carbonatites are petrogenetically evolved, with assemblages dominated by ankerite, siderite and Ba-Sr-REE carbonates. Microprobe analysis indicates baotite compositions more Nb-rich than previously recorded, with compositions close to Ba4[Ti3(Nb,Fe)5]Si4O28Cl. Ti must be partially replaced in both crystallographically-independent octahedral sites. Compositional zoning, and stoichiometric considerations suggest that the dominant octahedral substitution is the same as that described in rutile, namely 3Ti4+ ⇌ 2Nb5+ + Fe2+. Contrary to previous suggestions, Fe in the octahedral site should, therefore, be dominated by Fe2+.

The presence of baotite further documents the involvement of halogens in carbonatite magmas. In the New Zealand occurrences it is suggested that the chlorine originates from associated phonolitic magmas and is partitioned into carbonatite during liquid immiscibility.

Keywords

baotitecarbonatitefeniteNew Zealand
Type
Mineralogy
Information
Mineralogical Magazine , Volume 60 , Issue 400 , June 1996 , pp. 473 - 482
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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