uraninite

n.
A complex brownish-black mineral, UO₂, forming the chief ore of uranium and containing variable amounts of radium, lead, thorium, and other elements. It is isomorphous with thorianite.

[German Uranin (from New Latin ūranium, uranium; see uranium) + -ITE¹.]

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The chief ore mineral of uranium. Uraninite has the idealized chemical composition UO₂, uranium dioxide. Thorium and rare earths, chiefly cerium, are usually present in variable and sometimes large amounts. Lead always is present by radioactive decay of the thorium and uranium present. Complete solid-solution series extend between UO₂, ThO₂ (thorianite), and CeO₂ (cerianite). See also Radioactive minerals; Rare-earth elements; Uranium.

The color of uraninite is black, grading to brownish black and dark brown in the more highly oxidized material. The luster of fresh material is steel-gray. The hardness is 5½−6 on Mohs scale. The specific gravity of pure UO₂ is 10.9, but that of most natural material is 9.7−7.5.

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UO
Cubic -- hexoctahedral

Environment

Pegmatites and medium-temperature veins.

Crystal description

Two habits of this material are distinguished: crystals known as uraninite and a botryoidal variety with a radiating structure known as pitchblende. The crystals are cubes, octahedrons, and dodecahedrons. The less pure botryoidal type is more plentiful and significant, but is found at fewer localities.

Physical properties

Steely to velvety or brownish black. Luster submetallic, pitchlike, greasy, or dull; hardness 5-6; specific gravity 6.4-9.7; streak brownish black, grayish, or olive green; fracture conchoidal or uneven; cleavage none. Brittle; opaque.

Composition

Uranium dioxide; plus many other elements derived from the spontaneous breakdown of the uranium, the end-products of the series being helium and lead.

Tests

Infusible. Readily soluble in nitric and sulfuric acids, more slowly in hydrochloric. A drop of concentrated nitric acid left to dry on uraninite (free of calcite) evaporates to leave a fluorescent spot. The powder treated with a drop of nitric acid dries to a brilliantly fluorescent dot. Borax, sodium, and lithium fluoride beads are brilliantly fluorescent in ultraviolet light.

Distinguishing characteristics

Crystal form distinctive, but in rock it might be mistaken for microlite (which gives no fluorescent bead or test), magnetite (magnetic), and spinel (much lighter), among the cubic-system minerals. Other black minerals that might give trouble include tourmaline and cassiterite (light streak), columbite, and tantalite (no uranium test), and a whole series of dark, primary uraniumbearing minerals that would be hard to distinguish. Any of the uranium minerals placed on a photographic film in the dark for about 24 hours (or see your dentist for an x-ray film) would make self photographs.

Occurrence

Uraninite is a constituent of pegmatites; pitchblende is a vein mineral. The pegmatite occurrences are widespread but are economically of little importance. In these it is commonly altered to an orange and yellow, amorphous, greasy material (known as gummite) that sometimes surrounds a residual core of fresh black uraninite.

The important sources of uranium ore are the vein deposits, which have been subdivided into several types. Typical of the best are the silver-pitchblende veins of Jáchymov, Czechoslovakia, and Great Bear Lake, Northwest Territories. Pitchblende in the U.S. has come only from Colorado.

Good crystals and dendrites, altered in part to gummite, have come from the American pegmatites in New England and North Carolina. An unusual calcite pegmatite at Wilberforce, Ontario, has provided the largest known crystals, some of which reach 3 in. (7.5 cm) on a cubic edge. Usually uraninite crystals are small.

Remarks

Once considered almost worthless, pitchblende came into economic consideration first as a source of radium; with the atomic age it became about the most sought-after mineral in the world. Small quantities of uranium are widely distributed.

It has long been used to measure geologic time. Uranium atom after uranium atom transforms itself to lead, releasing helium. Now that the rate of radioactive decay is known, an analysis of the amount of lead or helium and the amount of remaining uranium immediately gives the time that has elapsed since the mineral came into being in the place where it was found. The only weak point in these analyses, which give the earth an indicated age of nearly five billion years, is an uncertainty about the possibility of a partial escape of some of the elements.

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