Tridymite mineralizers

The transformations are aided by or may requke the presence of impurities or added mineralizers such as alkaH metal oxides. Indeed, it has been suggested that tridymite cannot be formed at all in the absence of impurities, and some texts assert that pure Si02 occurs in only two forms quartz and cristobaHte... 

Another representation of the stabiUty relations of the siUca minerals is shown in Figure 4. This diagram, developed in the classical studies early in the twentieth century (51), illustrates the relationship of vapor pressure to temperature. It is assumed that vapor pressure increases with temperature and that the form having the lowest vapor pressure is the most stable. The actual values of the vapor pressures are largely unknown. Therefore, the ordinate must be considered only as an indication of relative stabiUties. This diagram does not show all the various forms of tridymite that have been identified. 

In addition to the three principal polymorphs of siUca, three high pressure phases have been prepared keatite [17679-64-0] coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stabiUty relationships of coesite, quart2, tridymite, and cristobaUte. A number of other phases, eg, siUca O, siUca X, sihcaUte, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a stmcturaHy unique fibrous form, siUca W. 

The a-form of each of the three minerals can thus be obtained at room temperature and, because of the sluggishness of the reconstructive interconversions of the -forms, it is even possible to melt -quartz (1550 ) and -tridymite (1703 ) if they arc heated sufficiently rapidly. The bp of Si02 is not accurately known but is about 2800 C. 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

They made several assumptions about which minerals could precipitate from the fluid. The alkaline lakes tend to be supersaturated with respect to each of the silica polymorphs (quartz, tridymite, and so on) except amorphous silica, so they suppressed each of the other silica minerals. They assumed that... 

A number of factors contribute to the disparity between the predictions of kinetic theory and conditions observed in the field, as discussed in Section 16.2. In this case, we might infer the dissolution and precipitation of minerals such as opal CT (cristobalite and tridymite, Si02), smectite and other clay minerals, and zeolites help control silica concentration. The minerals may be of minor significance in the aquifer volumetrically, but their high rate constants and specific surface areas allow them to react rapidly. 

Near the bottom of the profile, more tridymite forms than quartz, reflecting the former mineral s larger rate constant and specific surface area. 

Fig. 30.4. Changes in the volumes of minerals in the reservoir rock during the simulated alkali floods (Fig. 30.3) of a clastic petroleum reservoir using NaOH, Na2CC>3, and Na2SiC>3 solutions. Minerals that react in small volumes are omitted from the plots. Abbreviations Anal = analcime, Cc = calcite, Daw = dawsonite, Dol = dolomite, Kaol = kaolinite, Muse = muscovite, Parag = paragonite, Phlog = phlogopite, Qtz= quartz, Trid = tridymite.

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Silica Refractories. This type consists mainly of silica in three crystalline forms cristobalite [1446446-1]> tridymite [1546-32-3]> and quartz [14808-60-7]. Quartzite sands and silica gravels are the main raw materials, although lime and iron oxides are added to increase the mineralization of the tridymite and cristobalite. Uses include roof linings, refractories for coke ovens, coreless induction foundry furnaces, and fused-silica technical ceramic products. Consumption of silica refractories has declined dramatically since the 1960s as a result of the changes in the steel industry. 

Tridymite. Tridymite is reported to be the silica form stable from 870—1470°C at atmospheric pressure (44). Owing to the sluggishness of the reconstructive tridymite—quartz conversion, which requires mineralizers such as sodium tungstate, alkali metal oxide, or the action of water under pressure, tridymite may persist as a metastable phase below 870°C. It occurs in volcanic rocks and stony meteorites. 

The existence of tridymite as a distinct phase of pure crystalline silica has been questioned (42,58—63). According to this view, the only true crystalline phases of pure silica at atmospheric pressure are quartz and a highly ordered three-layer cristobalite having a transition temperature variously estimated from 806 250°C to about 1050°C (50,60). Tridymites are considered to be defect structures in which two-layer sequences predominate. The stability of tridymite as found in natural samples and in fired silica bricks has been attributed to the presence of foreign ions. This view is, however, disputed by those who cite evidence of the formation of tridymite from very pure silicon and water and of the conversion of tridymite M, but not tridymite S, to cristobalite below 1470°C (47). It has been suggested that the phase relations of silica are determined by the purity of the system (42), and that tridymite is not a true form of pure silica but rather a solid solution of mineralizer and silica (63). However, the assumption of the existence of tridymite phases is well established in the technical literature pertinent to practical work. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

Six different silica modifications were used vitreous silica, quartz, cristobalite, tridymite, coesite, and stishovite. Two of these—cristobalite and tridymite—were prepared from fine amorphous silica powder by tempering samples at 950°C. with 1% of a mineralizer (K2CO3 and KH2P04, respectively). Vitreous silica was obtained from fused rock crystals. All other samples were natural minerals pure specimens of Brazilian rock crystal were used as quartz coesite and stishovite were obtained as fine powders by isolation from Coconino sandstone of the Barringer Meteor Crater in Arizona (4). 

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

Diamond (D50) described the types of silica that can take part in ASR. They include quartz if sufficiently strained or microcrystalline, tridymite, cristobalite and glass or other amorphous forms, which occur in varying combinations in opals, flints, cherts and other rock types. Opals are especially reactive. Macroscopic, unstrained crystals of quartz appear to be Linreactive but are possibly not completely inert. Some silicate minerals and volcanic glasses may undergo reactions similar to ASR. 

What was believed to be a third form of Si02 tridymite is probably a solid solution of mineralizer and silica. 

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