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

¹ Crustal Geodynamics Group, School of Geography and Geosciences, University of St Andrews KY16 9AL, UK
² Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK
³ School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

^* Corresponding author

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: ferrotapiolite, bismuth, apatite, amblygonite-montebrasite, subsolidus replacement