Pyrochlore from Weathered Carbonatite at Lueshe, Zaire

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)2Nb206(OH,F) (where a general formula is Ae_xB206(OH,Fh_y.ZH20). 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 I,ueshe 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.

Pyrochlore chemistry from the Sokli phoscorite-carbonatite complex, Finland: Implications for the genesis of phoscorite and carbonatite association

“…General formula can be written as A 2-m B 2 O 6 Y 1-n ·pH 2 O, where (Lumpkin and Mariano, 1996). Structural formulae of the analyzed Sokli pyrochlore have been calculated based on a fixed total of 2.0 B-site cations, because large cation vacancies in the A-site could be produced by hydrothermal alteration or weathering (Wall et al, 1996;Williams et al, 1997). All pyrochlore grains analyzed in this study belong to the pyrochlore subgroup (Nb + Ta > 2Ti and Nb > Ta, Hogarth, 1977).…”

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…The conditions of formation of the amorphous, Fe-rich veins on the surface of porous pyrochlore differ from ones reported for weathered pyrochlore from Lueshe (Zaire) (Wall et al, 1996;Nasauri and Bilal, 2000) or from Mt. Weldt, Western Australia (Lettermoser and England, 1988 (Wall et al, 1996;Nasauri and Bilal, 2000).…”

“…Various studies show that the alteration of Ti-bearing pyrochlore supergroup minerals under natural or laboratory conditions, at temperatures 100-550 o C, led to the formation of various products that include TiO 2 (rutile, anatase), aeschynite, columbite, Nb-goethite, liandratite, uranpyrochlore, Nb-Ta oxide, and Y-REE phases (Lumpkin and Ewing, 1996;Wall et al, 1996;Nasraoui and Bilal, 2000;Xu et al, 2004;Geisler et al, 2005a,b;Pöml et al, 2007;Pöml et al 2011). In the samples analyzed here, no TiO 2 phases were observed.…”

“…Weldt, Western Australia (Lettermoser and England, 1988 (Wall et al, 1996;Nasauri and Bilal, 2000). Also, in the sample from Mt.…”

Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

“…As a first approximation, the B site remains stable (Nasraoui 1996, Wall et al 1996. Since the correct position of Fe and its valence state in the structure of weathered pyrochlore are generally unknown, all the Fe determined by chemical analysis was assumed to be in the Fe 3+ oxidation state and to be located at the B site.…”