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Silica polymorphic transformations

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. [Pg.926]

Once the amorphous silica has nearly disappeared, the cristobalite that formed early in the calculation begins to redissolve to form quartz. The cristobalite dissolves, however, much more slowly than it formed, reflecting the slow rate of quartz precipitation. After about 300 000 years of reaction, nearly all of the cristobalite has been transformed into quartz, the most stable silica polymorph, and the reaction has virtually ceased. [Pg.398]

At the temperature limits of their stability ranges, the main forms of silica interconvert. The transformations involve a change in the secondary (nonnearest-neighbor) coordination and require the breaking and reformation of Si—O bonds. The transformation processes, known as reconstructive polymorphic transformations (44), are slow, as shown by the fact that the high temperature polymorphs can persist outside their normal stability range. [Pg.472]

Silicalite-1 is the totally siliceous form of the zeolite MFl (ZSM-5), another silica polymorph. It belongs to the P2iln = Clh monoclinic space group (n. 14) with Z = 96. It transforms into an orthorhombic structure, belonging to the Pnma = Dlt space group (n. 62) between 350 and 363 K. Substituted silicalites such as ZSM-5 zeolite (see below) and Ti-silicalite adopt the orthorhombic structure even at room temperature, the transition temperature being strongly shifted to lower temperatures. [Pg.121]

Physical interactions between drug and excipient also can compromise quality. Adsorption of drug by microcrystalline cellulose resulted in drug dissolution being less than complete. Interaction between chloramphenicol stearate and colloidal silica during grinding led to polymorphic transformation. [Pg.1613]

TABLE 2. Volume changes on polymorphic transformations of silica... [Pg.222]

D iogenically precipitated silica is a metastable silica polymorph which must eventually alter to quartz under the earth s surface conditions. Present observations of deep-sea sediments suggest that this transformation may occur directly or through an intermediate, alpha cristobalite. Several models have been proposed to ascertain the rate at which these processes occur. This series of papers tests these models and offers simple but powerful methods for detecting changes in crystal form as a function of geologic age. [Pg.211]

Opal-CT is a poorly ordered silica polymorph composed of interstratified cristobalite and tridymite layers (Jones and Segnit, 1971, 1972) it is the mineral which is often misidentified as cristobalite in studies of the diage-netic transformations of silica. [Pg.472]

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. [Pg.337]

As already demonstrated, for relatively recent sedimentary rocks and weathering crusts (20-200 Ma), the characteristic sequence of silica transformation is silica gel —> opal-A —> opal-CT —> chalcedony —> quartz. This is a well studied, polymorphous transformation of the solid phase transition type (a-cristobalite —> a-quartz) (Plyusnina 1983,1986,1990). One aspect of the solid phase transformations is the gradual transition of one silica phase to another and the gradual transformation of morphological features down the stratigraphic column with increasing burial depth and age (similarly in weathering crusts). The observed sequential cristobalite-quartz ratio would be unlikely to occur in rocks with repeated dissolution and crystallization from solution (Plyusnina 1983). [Pg.122]

Holm, J.L., Kleppa, O.J., and Westrum, E.F. (1967) Thermodynamics of polymorphic transformations in silica Thermal properties from 5-1070 K and pressure-temperature stability fields for coesite and stishovite, Geomchim. Cosmochim. Acta, 31, 2289-2307. [Pg.37]

The change in composition of internal water is proposed as a method for approximate dating of agates (a variety of chalcedony) (Moxon, 2004). It has been demonstrated in geological environments that less stable silica polymorphs appear to have transformed over time to chalcedony and microquartz (Nash Hopkinson, 2004). [Pg.296]

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. ... [Pg.10]

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... [Pg.10]

Polymorphic transformations are accompanied by volume changes. Therefore, these transformations affect the thermal expansion coefficient. The values for the average volume expansion coefficient can be calculated using Equation 16.21 itself a for each phase here will have to be substituted by AV/VqAT. Figure 16.9 shows the variation of the linear thermal expansion of a silica containing quartz plus crystobalite phases and only a crystobalite phase. The material containing the two phases shows an increase in the value of the expansion coefficient at the transformation temperature. [Pg.318]

Effect of polymorphic transformation on linear thermal expansion coefficient. A silica containing crystobalite phase. B silica containing crystobalite and quartz phases. [Pg.320]

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. [Pg.823]

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... [Pg.590]

The polymorphism of silicas is based on different linkages of the tetrahedral [SiOj4- units (2). Quartz has the densest structure, and tridymite and cristobalite have a much more open structure. All three forms exist in a- and / -forms, which correspond to low- and high-temperature modifications, respectively. The a- and / -modifications differ only slightly in the relative positions of the tetrahedral arrangements. This similarity is evident from the fact that the conversion a (3 is a rapid displacing transformation that occurs at relatively low temperatures. Quartz is the most stable modification at room temperature all other forms are considered to be metastable at this temperature (2). [Pg.16]

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. [Pg.305]


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See also in sourсe #XX -- [ Pg.868 ]




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