Phase transformation tridymite

Intuitively, one might have expected radiation to increase rather than to decrease surface area because damage implies breaking up of crystals into smaller units. There is some evidence for macroscopic effects of this sort as a result of bombardment. The formation of tridymite crystals in silica xerogel (by neutron bombardment) was aptly termed 128) a brutal destruction of the original texture the measured surface area, however, decreased in this case. A phase transformation has also been induced in beryllia by electron bombardment in an electron... 

One of the best known examples of suspended transformation is found with the polymorphs formed by quartz [23]. The three principal polymorphic forms are quartz, tridymite, and cristobalite, which are enantiotropically related to each other. The ordinary transition point for the quartz/tridymite transition is 870°C, while the ordinary transition point for the tridymite/cristobalite transition is 1470°C. The melting point of cristobalite is at 1705°C, which exceeds all of the solid phase transition points. However, the phase transformations of these forms are extremely sluggish, and consequently each mineral form can be found in nature existing in a metastable form. 

Crystalline Silica Three principal polymorphic forms exist at atmospheric pressure. These are quartz, tridymite, and cristobalite. Quartz is stable below 870°C. It transforms to tridymite form at about 870°C. Tridymite is stable up to 1,470°C and transforms to cristobahte at 1,470°C. High cristobalite melts around 1,723°C. Other than these three polymorphs, there are also three high pressure phases of crystalline sihca keatite, coesite, and stishovite. 

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. 

The Si MAS-NMR spectrum narrows considerably upon transformation to the incommensurate OS phase (near 160°C), which has a single crystallographic site. The spectrum of this phase is characteristic of a non-linear modulation wave with low soliton density (cf Fig. 24c with Figs. 21 and 22). Both OP (210 to 320°C) and hexagonal (LHP above 320°C) tridymite phases give a single. 

It was found in previous studies [13,14] that when the temperature of calcination of freshly prepared iron phiosphate is raised, the surface area decreases markedly and the structure changes clearly, that is, amorphous phase FeP04 is transformed into tridymite type FeP04 at 400 to 500°C, and the tridymite type FeP04 is then transformed into quartz type FeP04 at a temperature above 500 to 550°C. 

Recently, a novel type of binder for catalyst shaping has been identified that allows for tailoring of the pore-size distribution in a different way. The material is an amorphous aluminum phosphate hydrate (APH) available as a powder, which is employed like conventional binders, for instance, in extrusion. In contrast to conventional particulate binders, however, calcination at 500 °C transforms APH via water evaporation, viscous sintering, and a phase transition into a dense, crystalline phase with the structure of tridymite. 

The different forms of quartz, tridymite, and cristobalite are transformed spontaneously with temperature so that from the standpoint of solubility there are only the three phases to be considered. 

It is also interesting to note that the a- to (3-quartz phase change can be seen at a temperature of about 600 °C and the transformation from (3-quartz to tridymite at about 800 °C, consistent with the literature [13]. 

Like the natural aluminophosphates variscite and metavariscite, these as-synthesized aluminophosphates are hydrates and contain octahedral aluminum where two of the coordination sites are occupied by water. Dehydration converts the octahedral aluminum to tetrahedral aluminum as shown in Fig. 3. This process, however, is limited in reversibility. For some structures such as HI, an irreversible topotactic transformation occurs resulting in the generation of a new phase, AIPO4-8 [30 - 35]. H2 can be reversibly dehydrated at room temperature but with further heat treatment converts to AlP04-tridymite [27]. H3, too, can be dehydrated to form AIPO4-C. Thermal treatment results in a topotactic transformation to AIPO4-D which can be hydrated to produce a different hydrate, H6. None of these phases have counterparts in the aluminosilicate zeolite system. 

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. ... 

Tridymite In his classical effort to determine phase equilibria relationships among the silica polymorphs, Fenner (1913) observed that tridymite could be synthesized only with the aid of a mineralizing agent or flux such as Na2WQj. If pure quartz is heated, it bypasses tridymite and transforms... 

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