April 20, 2011

Geology Word of the Week: U is for Uraninite

Posted by Evelyn Mervine

Botryoidal uraninite. Image taken from wikipedia commons here.

def. Uraninite:
A uranium-rich mineral with the formula UO2 (uranium oxide). Often, part of the uraninite is oxidized with the formula UO3. Uraninite is the primary ore for uranium and can also be mined for other elements such as radium, thorium, and lead, which are decay products of radioactive uranium. Uraninite deposits are generally a dark steel black with a slight metallic luster. The shape of uraninite is typically botryoidal (looks like a bunch of grapes) or amorphous, but rare cubic and octahedral crystals can form in certain environments. Uraninite often forms when hydrothermal circulation picks up uranium from a uranium-rich rock (such as granite or syenite) and concentrates this uranium in a hydrothermal ore deposit. The primary reason that uraninite is mined is to provide fuel for nuclear power plants.

Uranium can be found in almost all rocks. Uranium is found in at least trace quantities in rocks ranging from sedimentary limestones to granites to volcanic tephras. However, in most rocks the concentration of uranium is very, very low– parts per million or even parts per billion. That is, for every million atoms in the rock only one atom is uranium. Most rocks on Earth have ~1-2 parts per million uranium. Some uranium “enriched” rocks such as granite can contain as much as 50 parts per million uranium. Even though most rocks contain some uranium, it simply isn’t economical to mine uranium in most rocks since most rocks have very low concentrations of uranium. Instead, geologists must look for ore deposits in which uranium has been concentrated through a geological process, such as hydrothermal circulation. The uranium ore must then be processed and enriched before it can be used as nuclear fuel.

Uranium-rich ores are found and mined in many countries throughout the world Here is a nice pie chart from wikipedia (data taken from here):

Uranium mining by country. Taken from wikipedia commons here.

Here are some links if you want to learn more about uraninite and uranium mining:

Uranium Ore
Uranium Mining

Mineralogy of Uraninite:
Uraninite Mineral Data
The Mineral Uraninite

IAEA Website:
The Formation of Uranium Ore Deposits