Glass 6-cristobalite

Dear [33] described the mechanism of attack as the solution of first the glass, cristobalite, and other forms of silica in brick, followed by the solution of the finely divided mullite and clay body. The presence of well-crystallized mullite needles at the slag-interface indicated that the liquid at the contact surface was enriched with alumina. 

This is the process of the crystallization of one or more of the constituents of glass. Generally a glass is thermodynamically unstable with respect to these crystals, but at ordinary temperatures the crystallization rate is quite negligible. Crystallization may occur when the glass is worked at high temperature. The crystals which appear in a supercooled melt are not necessarily those of the stable solid phase at the temperatiure concerned for example, cristobalite can appear at temperatures for which tridymite is the stable crystalline... 

Figure 4 The structure of the two forms of silica cristobalite (crystalline, left) and silica glass (amorphous, right). (Reprinted with permission from Ref. 4.)...

X-ray diffraction measurements indicated that the zeolite rock consisted primarily of clinoptilolite (60-70%), volcanic glass (10%), feldspar (10%) and minor quantities of cristobalite, quartz and plagioclase (20%). Fig. 1 represents the XRD pattern of ODA-Clinoptilolite-rich tuff used for arsenate or chromate removal from aqueous solutions. 

Figure 1.50 X-ray diffraction patterns of vitreous silicon, crystalline silica (cristobalite), and sol-gel-derived silica. Reprinted, by permission from H. Scholze, Glass, p. 97. Copyright 1991 by Springer-Verlag.

Crystalline Silica. Quartz sand is of course the principal raw material for the production of glass (qv). Cristobalite and 3-quartz are used in glass ceramics (qv), ie, ceramics produced by the controlled crystallization of glass. Silica is a main constituent of ceramics (qv). For example, refractory silica brick containing small amounts of A O is used as roof brick for open-hearth furnaces at temperatures >1600° C (see Refractories). Silica sand or flour (ground quartz) is the raw material for soluble silicates, such as sodium silicate, which is consistently ranked as one of the top 50 U.S. industrial chemicals (98) (see Silicon compounds, synthetic inorganic silicates). 

Cristobalite can also form on vitreous silica at temperatures as low as 400°C when the pressure is equal to 35 MPa (<350 atm) and the glass is immersed in weak NaOH solutions (108). In stronger NaOH solutions, quartz is formed. The formation of the crystalline phases is a result of the hydrolysis of the anions present. No crystallization occurs with HF, H2S04, and H3P04 in KHS04 solutions or in pure water. 

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 listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

At 1973 K cristobalite is transformed to amorphous vitreous silica glass. The crystalline form involves a high degree of ordering in a dense structure. The active surface, which may participate in any chemical or physical interaction, is limited to the external surface of the crystalline particles. The specific surface area therefore is similar to the geometric surface. 

Cristobalite is transparent. We do not normally consider devitrified glass as a transparent material. However, once fused silica has cooled below 250°C, B-cristobalite is transformed into a-cristo-balite. This substance is the white opaque material we usually associate with devitrified silica. When fused silica is reheated into the devitrification range, the a-cristobalite turns back into B-cristobalite. However, because a-cristobalite has many fissures and cracks, the opacity remains when it is reheated back into B-cristobalite. 

Diamond (D50) described the types of silica that can take part in ASR. They include quartz if sufficiently strained or microcrystalline, tridymite, cristobalite and glass or other amorphous forms, which occur in varying combinations in opals, flints, cherts and other rock types. Opals are especially reactive. Macroscopic, unstrained crystals of quartz appear to be Linreactive but are possibly not completely inert. Some silicate minerals and volcanic glasses may undergo reactions similar to ASR. 

A 3-D framework analogous to the Si02 cristobalite stmcture is of course easy to build from elementary tetrahedra. Notice that, in the Ge/S system, the glass-forming ability is maximum for the composition GeS4 which corresponds to tetrahedra coimected by a S-S pairs bonds. 

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