PITCHBLENDE, or Uraninite, a mineral species consisting essentially of uranium oxide, of importance as a source of uranium and radium. It is a very heavy (specific gravity 9·0–9·7), compact mineral with a conchoidal to uneven fracture, and a brownish to velvet-black colour and pitchy lustre. Crystals are rare; they have the form of regular octahedra or less often of cubes. The hardness is 512, and the streak is brown with a greenish tinge. The mineral has been known to occur at Joachimsthal in Bohemia since 1727, and it was early called pitchblende, because of its appearance; but its true nature was not recognized until 1789, when M. H. Klaproth's analysis of it resulted in the discovery of the element uranium. Analyses of material from different localities exhibit wide variations in chemical composition. In addition to uranium oxides, there are thorium, cerium (and lanthanum), yttrium and lead oxides, each varying in amount from a trace up to 10%. Calcium, iron, magnesium, manganese, silica, water, &c., are also present in small amounts. The amounts of uranous and uranic oxides (UO2, 21–72; UO3, 13–59%) also vary considerably. The mineral is often described as a uranate of uranyl, lead, thorium and cerium; but in the least altered material from Branchville in Connecticut the uranous oxide predominates, whilst in altered specimens uranic oxide is in excess. In the closely allied mineral, thorianite, thorium predominates (ThO2, 76; UO2, 12%) Since the dioxides of uranium, thorium and cerium may be obtained artificially as cubic crystals, it seems probable that pitchblende consists of isomorphous mixtures of these dioxides, the uranic oxide being due to oxidation.
The radio-active properties of pitchblende are of special interest. The fact that this mineral is more strongly radio-active than metallic uranium led to the discovery in it of the elements radium, polonium and actinium. When pitchblende is ignited or dissolved in dilute sulphuric acid, a gas is evolved which consists largely of helium and argon: terrestrial helium was first recognized in this mineral.
The mineral occurs either as a primary constituent of granitic rocks or as one of secondary origin in metalliferous veins. Octahedral crystals (“cleveite” and “broggerite”) occur in the pegmatite veins of southern Norway, being occasionally found in the felspar quarries at Moss, Arendal and other places Crystals are found under similar conditions at Middletown and Branchville in Connecticut, Llano county in Texas (“nivenite”), Mitchell county in North Carolina, Villenveuve in Quebec, and other American localities. Thorianite, found as water-worn cubes in the gem-gravels near Balangoda in Sabaragamuwa province, Ceylon, has also no doubt been derived from crystalline rocks. On the other hard, the mineral found in metalliferous veins, and to which the name pitchblende is more properly restricted, never occurs as crystals, but as compact masses rendered more or less impure by admixture of other minerals, the specific gravity being sometimes as low as 6.5, thorium, cerium, &c., are absent, and radium and helium are present in smaller amounts. This variety occurs with ores of silver, lead, copper, nickel, cobalt, bismuth, &c, at Johanngeorgenstadt, Marienberg and Schneeberg in Saxony, Joachimsthal and Przibram in Bohemia, Rezbánya in Bihar Mountains in Hungary, Gilpin county in Colorado, St Just, in Penwith, Redruth, Grampound Road and elsewhere in Cornwall.
Often associated with pitchblende, and resulting from its alteration, is an orange-yellow, amorphous, gum-like mineral called gummite, which is a hydrous uranic oxide with small amounts of lead, calcium, iron, &c. (L. J. S.)