Geology of NORM

Igneous Rocks
Weathering of Igneous Rocks
Sedimentary Rocks
Shales
Sandstones
Carbonate Rocks
Metamorphic Rock
Soil

Igneous Rocks


The original sources of uranium-series, thorium-series, actinium-series, potassium and rubidium radioactivity in the terrestrial environment are the earth's crust and mantle (Table 4). As molten magma cools, silicate minerals are formed (magmatic differentiation). In the early stages of the cooling, the silicates tend to be mafic (those that are predominately iron and magnesium), and deficient in aluminum, silicon, sodium, and potassium. The mafic rocks are dark in color. As cooling and differentiation progress, the balance tends to reverse, the salic (containing mostly silicon-aluminum) igneous rocks as formed. They are generally lighter in color or speckled. Fig.2 shows a generalization of the process, known as the Bowen reaction series (Montgomery 1990). Neither uranium or thorium is compatible with the crystal structure of the major silicates. In addition, they are present in such small quantities as to have little tendency to form mineral in which they would be essential components. The result of this relationship is that the remainder of the magma cools to form miscellaneous and varied minor mineral which contain the uranium, thorium and other minor and trace elements. The last major silicates to crystallize are also those which contain most of the potassium and rubidium.


Weathering of Igneous Rocks


Mechanical (physical) and chemical processes break rock down into soil. Weathering plays a key role in this process.
Where mechanical processes dominate the breakdown, the separation usually occurs along mineral boundaries that leads to a separation of the major silicates from the minor ones containing the thorium and uranium. These minor minerals include zircon and monazite. They are stable and resistant to chemical decay and are often found as small individual grains.

Fig. 2. Bowen reaction series

Table 4. Crustal concentrations of terrestrial radionuclides


Rock Type Uranium Thorium Potassium Rubidium
Mafic

(Dark Colored)

 0.5 to 1 ppm 3 to 4 ppm 0.8% 40 ppm
Salic

(Light Colored)

 3 ppm 17 ppm 4% 170 to 200 ppm

Source: NCRP 1994.

Where chemical (or biological) action predominates, the thorium- and uranium-bearing minor minerals can give up their radionuclides to layers of cations in clay minerals. When the host rocks erode, the clay minerals containing with the adsorbed series radionuclides tend to be separated from the major minerals.
Therefore, if igneous rock is broken down to individual grains, the products end up as: