Uranium minerals of central Jordan

Abstract

Secondary yellow uranium minerals (uranyl vanadates) together with unique green smectites (Cr-rich smectite/volkonskoite) are hosted in altered varicoloured marble, chalk marl/travertine, and/or saprolite/top soil of central Jordan. Tyuyamunite Ca(UO₂)₂V⁵⁺₂O₈√3(H₂O)–strelkinite Na₂(UO₂)₂V₂O₈√6(H₂O) solid solution series (uranmica) are the major components and their composition reflects changes in the Ca/Na ratio of the circulating water. The Ca-end member tyuyamunite is also common in the secondary veins of the altered varicoloured marble. The Na-rich member (strelkinite) is dominant in the saprolite/top soil and its precipitation is related to continuous evaporation during seasonal dry periods. Carnotite K₂(UO₂)₂(V₂O₈).1–3H₂O is restricted to the secondary mineralisation in the veins of the altered varicoloured marble. Tyuyamunite–strelkinite phases are intermixed with the porous fine grained matrix (>50 μm to < 2 μm). The uranium phases have two stages of crystallisation. The earlier stage is composed of very fine crystallites ( < 1 μm) intermixed with the matrix. The coarser later stage (up to >50 μm) fills pores and weakness zones. The matrix is composed of U–F-bearing aggregates in intermixed calcite, carbonate-fluorapatite, and variable amounts of clay minerals dominated by smectite (montmorillonite), gypsum, barite, halite and fluorite. The porosity of the host rock controls tyuyamunite–strelkinite distribution. The green smectite fills voids and cavities of the altered marble, chalk marl/travertine and recent saprolite/top soil deposits. The chalk marl/travertine and/or saprolite/top soil are similar in their mineralogical composition. Tyuyamunite–strelkinite are present in all size fractions (>2 mm to < 2 μm) that are mainly composed of calcite, apatite and smectite (more in the < 2 μm size fraction). The ore beneficiation process could help in the liberation of tyuyamunite–strelkinite, but the ultrafine crystals ( < 1 μm) are expected to concentrate in the finest size fraction together with calcite, secondary apatite and more smectite. New primary calcium uranate phases (restricted to the varicoloured marble) were also identified – the calculated structural formulae are CaUO₄, Ca₂UO₅, Ca₃UO₆, Ca₃U₂O₉, Ca₄UO₇, Ca₅UO₈, and Ca₆UO₉.Crystallisation of calcium uranate phases is the result of oxidation of U⁺⁴ at high temperature as a result of combustion of phosphorus-rich bituminous marl into U⁺⁶ under high oxygen fugacity.