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Microlite-manganotantalite exsolution lamellae: evidence from rare-metal pegmatite, Karibib, Namibia

Published online by Cambridge University Press:  05 July 2018

J. R. Baldwin ,
P. G. Hill ,
A. A. Finch ,
O. von Knorring  and
G. J. H. Oliver
J. R. Baldwin*
Affiliation:
Crustal Geodynamics Group, School of Geography and Geosciences, University of St Andrews KY16 9AL, UK
P. G. Hill
Affiliation:
Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK
A. A. Finch
Affiliation:
Crustal Geodynamics Group, School of Geography and Geosciences, University of St Andrews KY16 9AL, UK
O. von Knorring
Affiliation:
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
G. J. H. Oliver
Affiliation:
Crustal Geodynamics Group, School of Geography and Geosciences, University of St Andrews KY16 9AL, UK
*
*Corresponding author
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Abstract

We have analysed a rare occurrence of orange-brown manganotantalite lamellae (visible in hand specimen), intergrown with microlite [(Ca,Na)2(Ta,Nb)2(O,OH,F)7], aggregates of ferrotapiolite, bismuth minerals and apatite to understand more about the mechanisms of crystal growth and secondary modification in Ta-rich minerals. The intergrowth occurs within amblygonite/montebrasite nodules near the quartz core of the highly fractionated rare-metal Li/Be/Ta pegmatite at Rubicon, Karibib, Namibia. Electron microprobe analyses show that manganotantalite lamellae are variable in composition. Primary microlite (Ta2O5 82%, 1.97 Ta a.p.f.u.) forms the matrix mineral between the lamellae. Textural relations suggest an exsolution origin for the lamellae. Manganotantalite is represented by three generations: (1) primary late magmatic; (2) disequilibrium exsolution lamellae; and (3) subsolidus replacement. Crystallization commenced with primary microlite and likely simultaneous intergrowth between ferrotapiolite and a first generation of late-magmatic primary manganotantalite with low Ta (1.1—1.5 a.p.f.u.). On cooling this was followed by exsolution of manganotantalite lamellae, generation (2) with low—medium Ta (1.27—1.7 a.p.f.u.). The replacement of microlite by a highly fractionated late-stage melt rich in Mn2+, Ca2+ with low Na+ finally produces a third generation (3) of manganotantalite with high Ta (1.72—1.99 a.p.f.u.) at the contact with microlite. Native bismuth and bismutite cut across microlite and pseudomorph lamellae as a final hydrothermal replacement. Apatite is ubiquitous at the contact with amblygonite. The stability field of microlite may be extended by incorporation of CaTa2O6-rynersonite and Ca2Ta2O7 — idealized, components in solid solution. However, rynersonite-CaTa2O6 with distorted octahedra has some structural templates which are similar to the structure of pyrochlore (microlite). Hence, via the perovskite/pyrochlore analogy, hypothetical exsolution of manganotantalite-type structures may occur from a microlite (pyrochlore) host by solid-state diffusion via metastable rynersonite-type intermediates. Such a mechanism has the potential to explain the crystallographically controlled intergrowth textures and the compositional heterogeneity.

Keywords

ferrotapiolitebismuthapatiteamblygonite-montebrasitesubsolidus replacement
Type
Research Article
Information
Mineralogical Magazine , Volume 69 , Issue 6 , December 2005 , pp. 917 - 935
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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