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Zirkelite from the Sebl'yavr carbonatite complex, Kola Peninsula, Russia: an X-ray and electron microprobe study of a partially metamict mineral

Published online by Cambridge University Press:  05 July 2018

A. G. Bulakh ,
A. R. Nesterov ,
C. T. Williams  and
I. S. Anisimov
A. G. Bulakh
Affiliation:
Mineralogy Department, St. Petersburg State Universit y, 199034 St-Petersburg, Russia
A. R. Nesterov
Affiliation:
Mineralogy Department, St. Petersburg State Universit y, 199034 St-Petersburg, Russia
C. T. Williams
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
I. S. Anisimov
Affiliation:
Mineralogy Department, St. Petersburg State Universit y, 199034 St-Petersburg, Russia
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Abstract

Zirkelite, a cubic mineral of general formula (Ti,Ca,Zr)O2−x occurs as an accessory mineral in phoscorites, carbonatites and associated rocks in the Sebl'yavr complex. After pyrochlore, it is the main mineral concentrating niobium in these rocks. Zirkelite forms platy crystals comprising polysynthetic octahedral twins, often having a complex skeletal internal structure within a cuboctahedral external morphology. The mineral is largely metamict, but after heating to 800°C it produces a cubic crystalline structure with ao = 5.157 ±0.006 Å. It is black to brownish-black in colour, VHN = 760−780 kg.mm−2, density = 4.27 g.cm−3, and reflectance = 12.5%.

Chemically, zirkelite is relatively enriched in Nb2O5 (up to 14.5 wt.%) and ThO2 (up to 7.7 wt.%), and it displays four compositionally-distinct zones which probably formed during primary crystallisation processes. It is patchily altered where it is associated with an unidentified Ba, Ti, Nb, Zr silicate phase which partly replaces and pseudomorphs it. Under the current IMA-approved nomenclature scheme, non-crystalline (metamict) minerals of the composition described here would normally be given the general species name zirconolite, with the name zirkelite confined to the cubic mineral. However, at Sebl'yavr, the name zirkelite is used because the mineral displays a well-defined cubic crystal morphology and has a cubic structure after heating.

Keywords

zirkelitezirconoliteSebl'yavrKola Peninsulacarbonatite
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 6 , December 1998 , pp. 837 - 846
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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