Cristobalite melting temperature

The low-pressure silica polymorphs include quartz, tridymite, and cristo-balite. The stable phase at room temperature is a-quartz or low quartz. This transforms to 3-quartz or high quartz at approximately 573°C at 1 bar. The transition from (3-quartz to tridymite occurs at 867°C and tridymite inverts to 3-cristobalite at 1470°C. P-Cristobalite melts to silica liquid at 1727 C. All three of these stable silica polymorphs experience displacive transformations that involve structural contraction with decreased temperature and all can be cooled stabily or metastabily to room temperature in glass-ceramics compositions. ... 

If these tetrahedra are arrayed in a regular and ordered manner, a crystalline structure is formed. There are three primary polymorphic crystalline forms of silica quartz, cristobalite (Figure 12.10), and tridymite, Their structures are relatively complicated and comparatively open—that is, the atoms are not closely packed together. As a consequence, these crystalline silicas have relatively low densities for example, at room temperature, quartz has a density of only 2.65 g/cm. The strength of the Si-O interatomic bonds is reflected in a relatively high melting temperature, 1710°C (3110°F). 

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... 

Melting point data were reviewed by Schneider (9) and adjusted to the 1948 International Temperature Scale. The adopted melting point for cristobalite is based on Greig ( ). A. H is calculated as the difference in A.H for liquid and crystal at... 

In contrast to the discrete molecules of carbon dioxide, silicon dioxide forms condensed, three-dimensional systems of indefinite extension which are high-melting solids. Silica has three forms, quartz, tridymite and cristobalite, each of which has a low-temperature (a) and a high-temperature (/3) modification. 

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. 

Quartz is also seen to be a non-equilibrium phase in another respect. At the firing temperature of porcelain it should transform to a more stable modification, which is cristobalite or tridymite under these conditions. However, the transformation is similarly very slow and cannot be completed during the firing cycle. Only in cases when the melt is saturated with quartz and its dissolution stops (or when the rate of dissolution is particularly low), distinct formation of a cristobalite layer occurs on the surface of quartz grains. 

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-... 

The primary phase fields that appear on the liquidus surface within the K20-Al203-Si02 system are shown in Fig. 6 [20]. The field of cristo-balite extends from 1470 10°C, the temperature of the boundary curve between cristobalite and tridymite to the melting point of cristobalite at 1713 + 5°C. 

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

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