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Zirconolite, chevkinite and other rare earth minerals from nepheline syenites and peralkaline granites and syenites of the Chilwa Alkaline Province, Malawi

Published online by Cambridge University Press:  05 July 2018

R. G. Platt ,
F. Wall ,
C. T. Williams  and
A. R. Woolley
R. G. Platt
Affiliation:
Dept. of Geology, Lakehead University, Thunder Bay, Ontario, Canada
F. Wall
Affiliation:
Dept. of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.
C. T. Williams
Affiliation:
Dept. of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.
A. R. Woolley
Affiliation:
Dept. of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.
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Abstract

Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims.

Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastnäsite. These are briefly described and microprobe analyses presented.

Keywords

zirconolitechevkinitemonazitefluoceritebastnäsiteChilwa Alkaline ProvinceMalawi
Type
Mineralogy
Information
Mineralogical Magazine , Volume 51 , Issue 360 , June 1987 , pp. 253 - 263
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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