Amorphous rare earth silicates

The multiphonon emission rates for the silicate glass in fig. 35.7 are much faster than for the crystals. Studies of rare earths in glass demonstrate that multiphonon relaxation is due predominantly to the highest frequency vibrational modes which are associated with the glass network former (Reisfeld, 1975). In both crystalline and amorphous rare earth hosts, materials having low vibrational frequencies generally have more fluorescing levels and hence more possibilities for laser action. 

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

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