The Rare Earths: Their Occurrence, Chemistry, and Technology

by Stanley Isaac Levy

CHAPTER V
THE OXIDES AND CARBONATES

(a) The Oxides

Uraninite or Pitchblende.

—Uraninite consists essentially of oxides of uranium (UO₂ + UO₃ = 75 to 85 per cent.), associated with thoria, zirconia, rare earths, beryllia, and oxides of lead. Traces of lime, iron oxides, silica, bismuth, and arsenic are also sometimes present, with water in widely varying quantities. Nitrogen and helium are always found in it, and, of course, radium. Groth regards pitchblende as uranous uranate U^iv(U^viO₄)₂, the uranium in the acidic radicle being hexavalent and in the basic radicle tetravalent, and in the latter condition partially replaced by lead, thorium, and rare earths.

Szilard[79] regards it rather as a loose compound or even a solid solution of oxides of thorium and uranium,[80] with small quantities of other oxides, he having obtained apparently homogeneous (though non-crystalline) bodies by dissolving thorium hydroxide in solutions of uranium salts and evaporating to dryness.

[79] Compt. rend. 1907, 145, 463.

[80] See under [Thorianite], infra.

The cubic form of the crystalline varieties has been taken as indicating that the mineral is really a spinel,[81] but it is difficult to see how the general formula of that group can be considered comparable to the uranyl uranate formula, UO₂,UO₃, for pitchblende.

[81] The Spinels are an isomorphous family of cubic minerals of the general formula R´´O,R´´´₂O₃, where R´´ = Be, Fe, Mg, Ca, etc., and R´´´ = Fe, Al, Cr, etc.

Crystals are rare, and belong to the cubic system, the common forms being the octahedron o {111} and the dodecahedron d {110}; the cube a {100} is sometimes present. The mineral is massive, usually botryoidal. The crystalline or primary form is black, with hardness 5¹⁄₂, sp. gr. 9·0 to 9·7; the altered varieties are grey to greenish- and brownish-black, sp. gr. 5·0 to 6·4.