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^{vi}O₄)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. It is infusible before the blowpipe, but readily soluble in nitric acid. The mineral occurs both as a primary and secondary constituent of rocks; as a primary mineral it is found in Norway, North Carolina, etc.; as a secondary species it occurs in the massive and hydrated form, with ores of lead, silver, tin, etc., in Saxony and Cornwall, and at the celebrated mine of Joachimsthal, in Bohemia. The latter deposits, consisting of the massive and altered varieties, for which the name Pitchblende is generally reserved, have been much used as a source of radium, especially those at Joachimsthal, and the Cornwall ore. Several varieties of uraninite have been distinguished by special names. Crystalline varieties from Anneröd and Arendal in Norway are known as Bröggerite and Cleveite respectively; Nivenite is a third form. In these varieties uranium oxides have been replaced to a considerable extent by the rare earths and thoria. An amorphous variety of doubtful composition, produced by alteration, is known as Gummite; Uranosphærite is a similar altered form.