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

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P.16<br />

4Ca 2A1 3Si30 12 (OH) 5~aA12Si208 + CaSi03 + Ca 2A12Si07 + 2H 20.<br />

zoisite anorthite wollastonite gehlenite<br />

Epidote appears readily on a laboratory time scale only at pressures in excess <strong>of</strong><br />

3 kilobars and temperatures in the range <strong>of</strong> 600 0 C (Deer, Howie, and Zussman 1962). Successful<br />

synthesis at atmospheric pressure by calcination or related techniques does not appear<br />

likely.<br />

P.2.1.4 Garnet Minerals<br />

The garnets are orthosilicates with the general formula<br />

X 3Y 2Si30 12<br />

where X = Mg, Fe 2+ or Ca; Y = Al, Fe 3+ or Cr 3+<br />

Although the garnets are dense and close-packed structures, the 8-coordinated X-cation<br />

site will accept large ions; Sr-substituted grossular (Ca 3Al 2Si30 12) may fit there. How-<br />

ever, grossular is best synthesized at temperatures in the range <strong>of</strong> 800°C under hydrothermal<br />

conditions with a water pressure <strong>of</strong> 2 kilobars. Attempts at lower pressure synthesis lead<br />

to a hydro-garnet, in which OH is substituted for the oxygen, or to mixtures <strong>of</strong> calcium sil-<br />

icates. In general, garnets are high-pressure phases in nature where they occur in metamor-<br />

phic rocks. Once formed, the garnets are resistant to weathering and appear as detrital<br />

minerals.<br />

P.2.1.5 Calcium Silicate Minerals<br />

Possible candidates among the calicum silicate minerals are limited, partly because <strong>of</strong><br />

the hydraulic nature <strong>of</strong> the anhydrous di- and tri-calcium silicates and partly because <strong>of</strong><br />

the poor resistance <strong>of</strong> the hydrated phases to mechanical degradation and their high reacti-<br />

vity under quite mild hydrothermal conditions. As with the pyroxenes to which they are<br />

related, the structures <strong>of</strong> possibly useful calcium silicate phases tend to be close-packed<br />

with limited possibilities for isomorphous replacement or crystalline solution (at least in<br />

the pure phases). Wollastonite (CaSi03) and rankinite (Ca 3Si 30 7 ) appear the only serious<br />

contenders in the group. Both form from oxides at 1200 C and represent the end members <strong>of</strong><br />

dehydration for hydrated calcium silicate phases. They show little reactivity at lower tem-<br />

peratures; in particular, neither is hydraulic. Strontium can replace calcium in both, making<br />

them possible hosts for that cation.<br />

Possibly <strong>of</strong> more potential use are compounds closely related to the calcium silicates<br />

but with <strong>of</strong>f-stochiometric composition. Bustamite [(Ca,Mn,Fe)Si031 and rhodonite<br />

[(Mn,Cu)Si0 3 , formally allied to wollastonite, have more "open" s ructures than wollastonite<br />

and may be able to accommodate a larger range <strong>of</strong> foreign ions in substitution. Synthe-<br />

sis and stability <strong>of</strong> these phases are similar to wollastonite.

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