Cristobalite 1334 INDEX

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

The various alkali metals may yield slightly different results in terms of catalyst performance. Lithium is slightly preferred over sodium as a melt-index promoter, and both may be preferred over potassium. Such differences have been reported in investigations on silica doped with 5 mol% of alkali metal and calcined at 700 °C. Lithium caused crystallization to form quartz whereas sodium and potassium caused conversion to cristobalite [477,609-612],... 

The topography of Si02 is determined by the index of face (111) and face (100) in / -cristobalite the silanol number of... 

Silica X is a microcrystalline form obtained as spherical aggregates of radial fibers up to 12 microns in diameter by heating pure amorphous hydrated silica ( silicic acid ) with 2% KOH solution in sealed tubes at 150 C for a few weeks. The refractive index of 1.484 0.004 is close to that of cristobalite. It is anhydrous, maintaining its structure up to about 600 C, above which it is converted to cristobalite (71a, 71b). 

The foregoing minerals are varieties of quartz. Opal is a cryptocrystalline variety of cristobalite, somewhat hydrated. The striking colors of opal are caused by Bragg diffraction (Appendix IV) of visible light by spheru-lites of cristobalite about 300 nm in diameter that have settled into a close-packed array and have been cemented together by a silicious cement with index of refraction different from that of the spherulites. 

The coverage value of 4.9 OH/nm corresponds quite closely to the value expected if on average the amorphous silica surface was represented by the < 111 > face of cristobalite which has a density and refractive index similar to that of amorphous silica [1]. Figure 6 shows the atomic arrangement of... 

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