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

P.17
Although the pure di-calcium silicates must be ruled out, appreciable amounts of
lanthanide solution occurs and stabilizes the non-hydraulic, Y-Ca 2SioO 4 form. This phase
may act as strontium and a lanthanide host, but studies are needed to define solubility
limits and the stability of material.
Recently, Scott (1976) described the crystal structure of a hydrated potassium-calcium
silicate, miserite [KCa 5 (Si0O 22 )(OH)F], which appears capable of incorporating a wide vari-
ety of cations into a vacant site and "locking" them there. The mineral occurs with aegirine
and orthoclase, sometimes with wollastonite; it appears to be geologically stable and
a potentially useful host for a wide range of cations if some way to incorporate them into
structure can be found. Studies of the synthesis and stability of miserite could prove
fruitful.
P.2.1.6 Layer Silicate Minerals
The layer silicate minerals include the micas, the clays and the chlorite families.
The mica family has the general formula WO_ 1Y 2-3(Si 2A1) 40 10 (OH) 2 where W and Y have the same
meaning as in the pyroxene and amphibole discussion. The same range of ionic substitutions
occurs as in the amphiboles and pyroxenes. Fluorine and less commonly Cl- and S 2- may sub-
stitute for the (OH). Biotite is commonly reported from granites and pegmatites, which contain
traces of rare earth elements, but these traces can usually be attributed to xenotime
(Y...)P0 4 inclusions rather than to being incorporated into the mica structure directly.
The remarks also pertain to the other groups of layer silicates as far as ionic substitutions
are concerned. Because chlorites and clays may have layer units with residual elec-
tronic charges, some ions may be adsorbed on the surfaces. Interlayer ions may be easily
exchanged. The permanence of these attachments, however, is poor and the materials cannot
be considered potential repository phases.
P.2.1.7 The Melilite Minerals
The common melilites are a solid solution
Ca 2MgSi 207 - Ca 2Al 2SiO 7
akermanite gehlenite
in which magnesium is gradually replaced by aluminum. The entire series can be prepared
synthetically by dry-firing--that is, calcination techniques at temperatures in the range
of 1000 to 1200 0 C. The minerals as found in nature in high temperature, low pressure envi-
ronments and synthetically in slags are related materials. They appear to be stable under
ambient conditons. Strontium analogs can be made and this mineral series is a potential
host for 90 Sr.
P.2.1.8 Feldspar Minerals
The feldspar minerals are the most abundant mineral group on the earth and a major con-
stituent of granite rocks, but they are remarkably simple in chemistry. They have the for-
mula (K,Na,Ca,Ba),A1 _2Si 2_308 with almost no other chemical substitutions allowed. Boron