Cristobalite, inversion

Payne J. A. and Drever R. (1965). An investigation of the beta decay of rhenium to osmium with high temperature proportional counters. Ph.D. diss.. University of Glasgow. Peacor D. R. (1973). High temperature single-crystal study of the cristobalite inversion. Zeit. Krist., 138 274-298. 

High-temperature electrolysis of tridymite brings about migration of impurities towards the cathode, while tridymite in the vicinity of the anode is converted to quartz below 1050 °C and to cristobalite above 1050 °C this temperature is approximately equal to that of quartz-cristobalite inversion, according to data of other authors. 

Peaoor, D.R. (1973) High-Temperature Single-Crystal Study of the Cristobalite Inversion, Z. Kristallogr., 138, 274-298. 

Peacor, D. R. High-temperature single-crystal study of the cristobalite inversion. Z. Kristallogr. 138, 1973, 274-298. 

Temperature Limit Like any other ceramic material, many factors affect the maximum use temperature of high purity silica products. In general, 2000°F is the highest temperature limit for cyclic service. When the temperature goes above 2000°F, the vitreous/fused silica grains will crystallize to cristobalite and quartz. If the operating temperature is then cycled, the various silica inversions can take place which will tear the brick apart. When operation is restricted to continuous service only, then the maximum use temperature is approximately 3000°F. 

Figure 8 indicates that the conversion of quartz into cristobalite proceeds very slowly even at 1500 °C, i.e. almost 500 °C above the equilibrium inversion point, so that it is not surprising that quartz is present in ceramics even after high-temperature firing. 

The following sequence of inversions takes place during gradual heating up of low-temperature quartz at 573 °C, jS-quartz is very rapidly inverted to a-quartz, which is stable up to about 1025 °C when it has high purity (content of impurities < < 10 %). Itis then converted to cristobalite. If the quartz contains more impurities (solid solutions), then at about 870 °C a-quartz is converted to a-tridymite which in turn is transformed to a-cristobalite above 1470 °C, Non-equilibrium fusion of quartz at temperatures above 1400 —1450 °C was often observed cristobalite is then formed secondarily from this melt. 

During cooling, the sequence of inversions is different tridymite and cristobalite are not reconverted to quartz under normal conditions however, they are subject to very rapid displacive transformations to y-tridymite and jS-cristobalite respectively. The high-temperature a-forms cannot be undercooled. 

Silica crystallizes from sodium silicates in three forms, cristobalite, tridymite, and quartz. The inversion temperatures are 1470 and 870°C. Cristobalite, the high-temperature modification, melts at 1713°C. The cristobalite liquidus decreases from the melting point of SiOz to the inversion point (located at 88.7 wt % Si02) between cristobalite and tridymite. The tridymite liquidus then descends from this point and meets the liquidus curve of quartz at 75.5 wt % Si02 (870 10°C). The tridymite liquidus extends metastable below 870 to 793°C ending at the disilicate-... 

Whilst the inversion temperature of quartz does not appear to vary appreciably, that of cristobalite has been found to depend on its previous history and may vary, as indicated, over a range of temperature. This variation is believed to be caused by differences... 

Tridymite also undergoes a series of inversions, the number and type of which vary from one specimen to another. Because the number of modifications is greater than with quartz or cristobalite, Roman numerals are now used to distinguish them and have replaced the older a - / notation. Thus, a typical specimen of tridymite undergoes the following inversions ... 

The variation in the coefficient of thermal expansion of quartz and cristobalite with temperature reveals very strikingly the a-P inversions, and is best seen by plotting the percentage volume expansion for each mineral against the temperature (Figure 9). Owing to the difficulty of obtaining pure tridymite its expansion... 

At the inversion temperatures there is clearly a marked increase in the rate of expansion for each mineral, particularly for the a-p quartz change at 573°C and for the a p cristobalite change at 220°-260 C. These sudden expansions are the cause of spalling in silica refractories. 

CU2O (cuprite), Ag20 with sp -hybridized oxygen atom and sp-hybridized Cu or Ag atom. The structure is of inversed cristobalite. 

Conversion. A change in crystalline structure on heating that is not immediately reversible on cooling. The most important example in ceramics is the conversion of quartz at high temperature into cristobalite and tridymite (cf. inversion). 

The room-temperature form, a-quartz (sp. gr. 2.65) undergoes a reversible crystalline change to p-quartz at 573°C this inversion is accompanied by a linear expansion of 0.45 %. At 870 C quartz ceases to be stable but, in the absence of fluxes, does not alter until a much higher temperature is reached, when it is converted into cristobalite and/or tridymite, depending on the temperature and nature of the fluxes present. 

The phase transition temperature between low and high modifications of cristobalite does not appear to be constant, but a typical temperature is around 215°C. The transition is accompanied by large changes in thermal expansion. The z- and c-axis of a-cristobalite increase rapidly at rates of 9.3 X 10 and 3.5 X 10 A K respectively whereas in p-cristobalite, a expands at only 2.1 X 10 A This behavior translates into very large, spontaneous strains of -1% along /z-axis and -2.2% along c-axis during inversion. 

While tridymite and cristobalite may exist for indefinite periods of time at room temperature, tire low-temperature alpha-quartz is believed to be the form of silica truly stable at these temperatures. A considerable expansion accompanies the conversion of a-cristobalite to /J-cristobalite at 220-275°C, and a sudden expansion of -2.2% occurs during the inversion of a-quartz at 573°C. The conversion of quartz to tridymite at 870°C is accompanied by an expansion of -15%. 

The low-temperature inversions associated with tridymite and cristobalite are as follows -tridymite jS -tridymite —- - j82-tridymite... 

There appears to be an inverse relationship between the stability of the silica polymorphs and their solubility in water thus, opal is more soluble than quartz (BeleVtsev et al. [I960] Millot [I960]), and since many of the reactions of silica and silicates in soils are surface controlled, particle size must be considered (Jackson et al. [1949]). Because of the dependence of solubility on particle size, and hence the difficulty of determining the absolute solubility of the silica polymorphs, comparison cannot be made without qualification. Values of 120 to 140 ppm at 25°C have been quoted for the solubility of noncrystalline silica in water by Alexander et al. [1954], who also observed that the solubility rises sharply above pH 9, varies linearly with temperature, and is influenced by particle size and the number of internal OH groups. The solubility of cristobalite (prepared by heating 100 to 140 mesh quartz grains... 

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