Management of Commercially Generated Radioactive Waste - U.S. ...

P.1.1.2 Use of Geologic Data
P.6
Because geologic time spans the lifetimes of the radionuclides of the critical ele-
ments, it is very logical to use nature as a laboratory and examine conditions of stability
of minerals that may contain the critical elements. In general one recognizes three main
geologic environments (igneous, sedimentary, and metamorphic) and asks which mineral phases
may exist in each environment and what happens to a mineral grain as it sees a change in its
environment. Minerals of the igneous environment see extreme temperatures (and pressures)
such that they have crystallized from a melt or a fluid derived from a melt (pegmatites and
hydrothermal deposits). The sedimentary environment includes the effect of exposure to the
atmosphere and running water and the physical effects of separation and movement of mineral
grains. The metamorphic environment involves changing pressure, temperature and pore fluid
conditions inducing mineral changes in situ.
As one identifies mineral species that may be potential repository compounds, a test
of their stability is to determine the geologic environments under which they can endure.
If any modifications in the mineral phase do occur, then the time frame of the modifications
can also be deduced. The best test of a mineral's stability is to determine the range of
changes through which it can exist.
Many of the minerals that are potentially interesting host phases form initially in the
igneous environment. Feldspars, feldspathoids and micas crystallize directly from the melt.
Many others are pegmatitic in origin, especially those containing rare earth elements (REE).
This information implies conditions that may be necessary to form the phase desired. It may
not be the only condition under which the compound will form.
After the compound has formed, the question of what happens to it as the conditions
change may be answered. Because stability is the main question, one asks what phase may
endure weathering and erosion unchanged, and what new phases are formed if changes do occur.
Many minerals survive the rigors of weathering and erosion and these are ultimately col-
lected in detrital deposits. When the detrital deposit has an economic value it is called
a placer. These minerals are usually of high density and chemical resistance. Other min-
erals, called detrital-heavy minerals, may not survive the entire erosion cycle but persist
for quite some time. Detrital-heavy minerals may last sufficiently long to allow included
radionuclides sufficient time to decay. Therefore, it is useful to identify the placer min-
erals and other detrital-heavy minerals.
The Placer Minerals
Table P.1.4 identifies the minerals that have been recognized in placer deposits.
These minerals are characterized by high densities and chemical and physical resistance.
All the noble metals--platinum, iridium, palladium, gold--are known to occur as placer min-
erals. Many oxides containing lanthanides as well as carbonates, phosphates, tungstates