Cristobalites

Cristobalite from quartz by heating in the presence of catalyst.s 

Cristobalite (see Section 5.8.3.1) is produced in large quantities by thermal treatment of quartz sand. The transition temperature, generally ca. 1100°C, can be reduced by adding catalysts. The most important advantage of cristobalite, in comparison with quartz, its much higher whiteness. 

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Table 3. Thermodynamic Properties of Quartz, Cristobalite, and Liquid Si02...

Fig. 7. Structure of high cristobalite, where O and 0 represent Si and O, respectively (44).

Fig. 4. X-iay diffraction patterns using a Cu anode (a) ciystalline quartz sand, and (b) cristobalite.

Transmission electron micrographs show hectorite and nontronite as elongated, lath-shaped units, whereas the other smectite clays appear more nearly equidimensional. A broken surface of smectite clays typically shows a "com flakes" or "oak leaf surface texture (54). High temperature minerals formed upon heating smectites vary considerably with the compositions of the clays. Spinels commonly appear at 800—1000°C, and dissolve at higher temperatures. Quartz, especially cristobalite, appears and mullite forms if the content of aluminum is adequate (38). 

Respiratory fibrogens Fibrogenic dusts e.g. Free crystalline silica, (quartz, tridymite, cristobalite), asbestos (chrysotile, amosite, crocidolite etc.), talc. 

A crystalline form of free silica, extremely hard and inert chemically very resistant to heat. Quartz in refractory bricks and amorphous silica in diatomaceous earth are altered to cristobalite when exposed to high temperatures (calcined). Cristobalite is extensively used in precision casting by the hot wax process, dental laboratory work, and certain speciality ceramics. 

A soft, gritty amorphous silica composed of minute siliceous skeletons of small aquatic plants. Used in filtration and decoloration of liquids, insulation, filler in dynamite, wax, textiles, plastics, paint, and rubber. Calcined and flux-calcined diatomaceous earth contains appreciable amounts of cristobalite, and dust levels should be the same as for cristobalite. 

Silica [14808-60-7], crystalline (inhaled in the form of quartz or cristobalite from occupational sources) (Vol. 68 1997) Solar radiation (Vol. 55 1992)... 

Cristobalite, see Silica, crystalline Crocidolite, see Asbestos Crotonaldehyde C0.3 1 7 (trans)... 

Silica, Crystalline (Respirable) Cristobalite Quartz Tridymite Tripoli... 

Cristobalite in respirable airborne dust Lab method using X-ray diffraction (direct method) 76... 

Although natural quartz, cristobalite and opal are used as fillers, only synthetic products (fumed and precipitated silicas) find use as fillers in rubber base adhesives. 

Air Samples Respirable dust samples are analyzed for quartz and cristobalite by x-ray diffraction (XRD). XRD is the preferred analytical method due to its sensitivity. 

Approximate Values in Units of Percent The particle size distribution in a total dust sample is unknown and error in the XRD analysis may be greater than for respirable samples. Therefore, for total dust samples, an approximate result is given. (4) Nondetected A sample reported as nondetected indicates that the quantity of quartz (or cristobalite) present in the sample is not greater than the... 

Bulk Samples Bulk samples must be submitted for all silica analyses. They have two purposes (1) For laboratory use only, to confirm the presence of quartz or cristobalite in respirable samples, or to assess the presence of other substances that may interfer in the analysis of respirable samples. (2) To determine the approximate percentage of quartz (or cristobalite) in the bulk sample. A bulk sample submitted "for laboratory use only" must be representative of the airborne free silica content of the work environment sampled otherwise it will be of no value. The order of preference for an evaluation is ... 

A bulk sample is the last choice and the least desirable. It should be submitted "for laboratory use only" if there is a possibility of contamination by other matter. The type of bulk sample submitted to the laboratory should be cross-referenced to the appropriate air samples. A reported bulk sample analysis for quartz (or cristobalite) will be semi-quantitative in nature because (1) The XRD analysis procedure requires a thin layer deposition for an accurate analysis. (2) The error for bulk samples analyzed by XRD is unknown because the particle size of nonrespirable bulk samples varies from sample to sample. 

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

When water freezes the crystalline form adopted depends upon the detailed conditions employed. At least nine structurally distinct forms of ice are known and the phase relations between them are summarized in Fig. 14.9. Thus, when liquid or gaseous water crystallizes at atmospheric pressure normal hexagonal ice If, forms, but at very low temperatures (—120° to — 140°) the vapour condenses to the cubic form, ice Ic. The relation between these structures is the same as that between the tridymite and cristobalite forms of SiOa (p. 342), though in both forms of ice the protons are disordered. 

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