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

P.15
The reported rare earth content of any pyroxene is never greater than trace quantities,
and these are probably due to minute inclusions of other rare earth minerals.
Pyroxenes form easily in both dry and hydrothermal systems, and they are common reac-
tion products in many silicate experiments. In studies on the decomposition of nuclear
waste products in glass under mild hydrothermal conditions, pyroxene was a common end product.
Even with the presence of all radionuclides at moderate concentration levels, none
of them was detected in the pyroxene phase.
P.2.1.2 Amphibole Minerals
The general formula of the amphibole minerals is WO_1X 2Y 5(Si 2Al) 80 22 (OH) 2. The
X and Y sites are essentially identical with those so labeled in the pyroxene minerals. The
limits on ionic substitutions are the same also. The W site, which is not always occupied
in amphiboles, accepts low-charge cations in the ionic radius range 0.95x to 1.35 x A. These
are usually only Na + and K + , and no other ions are known as substitutes. Amphiboles have
sometimes been called "nature's waste-baskets" because the W, X and Y sites can accept so
many elements, but the structures are not suitable for any of the critical radionuclides
except possibly Tc+ 4 . The remarks concerning Tc +4 are the same as for the pyroxenes dis-
cussed above.
The synthesis of amphiboles is not favorable for them to be considered as potential
repository phases. Because the minerals are hydrous, water pressures must be maintained
during the synthesis. This, in turn, requires that hydrothermal methods be used. Volcanic
rocks rarely contain amphiboles because the water leaves the lava when it reaches the sur-
face. Amphiboles that survive are usually formed in the magma chamber before eruption.
P.2.1.3 Epidote Minerals
The compositional formula for the epidote minerals is X 2Y 3Z 3 (0,OH,F)13 in which
X = Ca, Ce 3+ , La 3+ , y3+, Th, Fe 2+ Mn 2+ , Mn 3+
Y = Al, Fe 3+ , Mn 3 , Fe 2+ , Ti
Z = Si, Be.
The compositions of epidote minerals that occur commonly are:
zoisite/clinozoisite Ca 2A1 3Si30 12 (OH);
epidote Ca 2FeA12Si30 12 (OH)
piemonite Ca 2 (Mn,Fem,Al) 3Si30 12 (OH)
allanite (Ca,Ce,La,Y) 2(Mn,Fe+ 2 ,Fe+3,Al) 3Si30 12 (OH)
Allanite is resistant to weathering and appears as a detrital mineral.
The large X-cation site in epidote is suitable for incorporating 90Sr, rare earths,
and possibly actinides in synthetic analogs of allanite. However, epidote is not suitable
as a nuclear waste host because of the difficulty in synthesizing the mineral. All of the
epidote minerals are stable at low temperatures and modest to high pressure. At high tem-
perature (greater than 600 to 700 0 C) the epidotes dissociate according to the reaction