Pyrochlore from Weathered Carbonatite at Lueshe, Zaire

F. Wall, C. T. Williams, A. R. Woolley and M. Nasraoui
Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Département de Géochimie, Ecole des Mines de St Etienne, 158, Cours Fauriel 42100, St. Etienne, France

Abstract: A detailed study of weathered pyrochlore in the laterite above carbonatite at Lueshe, NE Zaire, has been made in order to determine its chemical and textural variations. Pyrochlore in fresh carbonatite at Lueshe is close to an ideal formula of (Ca,Na)2Nb2O6(OH,F) (where a general formula is A2−xB2O6(OH,F)1−y·zH2O. The first and principal change on weathering occurs at the base of the profile and involves the leaching and partial exchange of A cations together with hydration. This change appears common to weathered pyrochlore worldwide. As a result weathered pyrochlore at Lueshe has a large apparent A cation deficiency with A totals between 0.25 and 0.59. The B cations remain stable. Abundant kalipyrochlore is unique to Lueshe and is thought to be related to the abundance of potassium feldspar in the fresh carbonatite, showing that the actual composition of weathered pyrochlore is a characteristic of a particular deposit. Weathered profiles at Lueshe are not simple trends from the least to most leached compositions. Further factors including variation in whole rock mineralogy and chemistry, and cation exchange and uptake are responsible for local concentrations of strontio-, bario- and calcium-rich, sodium-poor pyrochlore in the ore body, as well as rims of ceriopyrochlore on kalipyrochlore. The most important textural relationship in the Lueshe pyrochlore is the intimate intergrowth with crandallite in the most weathered parts of the laterite. Although pyrochlore persists throughout the weathering profile, niobium-beating goethite is thought to represent the final product of pyrochlore breakdown.

Keywords: pyrochlore • carbonatite • weathering • kalipyrochlore • Zaire

Mineralogical Magazine; October 1996 v. 60; no. 402; p. 731-750; DOI: 10.1180/minmag.1996.060.402.03
© 1996, The Mineralogical Society
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