Minerals are complex structures containing many different substances. The bonds between the substances within the mineral can be very strong. Uranium is mostly found in minerals as triuranium octaoxide (U3O8), the most stable form of uranium in nature.

The minerals are usually embedded in hard rock or sandstone (?host rock?), covered with earth and vegetation. Uranium is also found in phosphate rock and in lignite and monozite sands.

On average, uranium occurs in the earth crust in about 3 ppm ? comparable to the size of a tablespoon of uranium in a truckload of dirt. All over the world, deposits with more concentrated uranium minerals can be found. Uranium is not found uncombined in nature.

Uranium is a naturally occurring element, which was present when our planet first formed.

Since then uranium has been subject to decay. That is why uranium in minerals always is accompanied by its also radioactive decay products. These are for example radioactive lead, polonium, bismuth, thorium, radium, protactinium and radon gas.

Other toxic elements like nickel, cadmium, molybdenum, vanadium, arsenic and mercury are often also present in uranium minerals.

Uranium is a radioactive and chemically toxic element, harmful to living beings. But in these natural deposits the uranium is held in a relatively safe location, buried deep in the earth. The underground deposits give little chance for the minerals to spread. Also, the forces within the mineral keep the uranium in place.

Radioactive radon gas can be a danger in the surroundings of uranium deposits. Radon gas can escape from the rocks when there are fissures. Radon has a short physical half-life (3.8 days). When radon gas is formed and becomes airborne, its solid radioactive decay products will be deposited downwind. If the radon gas can not escape the rock, its solid decay products will remain within the rock.

Uranium rock has eroded for billions of years, for example from deposits located at the surface of the earth. The eroded mineral uranium contributes largely to the so-called 'natural background radiation'. A worldwide varying but small trace of mineral uranium is found in water, soil, air and all living beings. Mineral uranium gets into the human body mainly through eating and drinking. Although never without danger, the human digestive system fortunately is very effective against mineral uranium. About 99% of the ingested mineral uranium is excreted in faeces within 24 hours.