Role of geochemical alteration on the formation of secondary Zr- and U-bearing minerals in El Atshan trachyte, central Eastern Desert, Egypt

Abstract

El Atshan area, situated in the central Eastern Desert of Egypt, is a good location for studying the influence of high -and low -T alterations on the formation of Zr -and U -bearing minerals within a trachyte sill. These minerals are mainly represented by an unidentified secondary Zr -rich silicate mineral, betafite, and liandratite. The unidentified Zr -rich silicate mineral is considered to be an alteration product of the precursor zirconolite during high -T alteration. This is indicated by the changes in the composition marked by an increase in the amount of hydration (H 2 O), Si, and U and a decrease in the amount of Ca, Ti, Nb, and Fe. The morphology of this unidentified mineral is similar to that of zirconolite. For seven oxygen atoms, the calculated formula of the unidentified Zr -rich silicate mineral is (Si . With more extensive alteration, it was found that the unidentified secondary Zr -rich silicate mineral was unstable and it underwent re -equilibration with U -rich fluid, which led to alterations in betafite. The two possible mechanisms responsible for the alteration of the unidentified Zr -rich silicate mineral to betafite are as follows: (1) the dissolution of the unidentified Zr -rich silicate mineral and precipitation of betafite and (2) the ion -exchange between partially to fully amorphized zones and the U -rich fluid. Such alteration is indicated by a marked increase in the amount of U, Ti, and Nb and a decrease in the amount of Zr, Si, Ca, P, Y, and ΣREE.

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

“…Since its first description as a new mineral species, liandratite has only been mentioned by several authors (Lumpkin and Ewing 1996;Lumpkin 2001;Kristiansen 2006;Abd El-Naby 2009;Galliski et al 2009;Matyszczak 2013Matyszczak , 2014, while chemical data seem to have been presented only by Lumpkin and Ewing (1996). The latter authors found liandratite as a weathering product of betafite (□,U 4+ ) 2 Ti 2 O 6 (O,OH,F) and analyses revealed that the chemical composition of liandratite is no less complex than those of the other Nb-Ta-Ti oxides.…”