Keatite

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. 

Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Vitreous Si02 oeeurs as teetites, obsidian and the rare mineral leehatelierite. Synthetie forms inelude keatite and W-siliea. Opals are an exeeedingly eomplex erystalline aggregate of partly hydrated siliea. 

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. 

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

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. 

If the catalyst is placed inside the membrane tubes and also the separative layer is coated at the inside of the tube, any compounds such as potassiumoxide from the catalyst might react with the silica separative layer to form keatite. This will destroy the molecular sieving properties of the silica toplayer, see paragraph 3.1. Additionally there is a risk that catalyst loading will damage the membrane layer. 

Much attention should be paid to the choice of the catalyst for the reaction. Various catalysts of which M0S2 is the most common, are possible. In a separate subtask, the impact of the used catalyst on the stability of the membrane should be studied. E.g., alkali doping might have a negative effect on the stability of the silica layer due to keatite fomation as described in this thesis. 

Influence of potassium from the catalyst on the stability of the silica layer (possible keatite formation). 

How do these unconventional ideas link with the standard view of a solid as a close packed array of atoms Evidently most of the frameworks discussed above cannot be so characterised. The two-dimensional hyperbolic picture does break down for very dense structures. Thus the densest four-coordinated silicate, coesite, violates this universality (see Fig. 2.12). (Its ring size is less than that of trid5m[ ite, cristobalite, keatite or quartz, in spite of its higher density.) This polymorph is too dense for a two-dimensional description to be useful and the Aree-dimensional description takes over. The notion of intrinsic curvature is less rigid for silicates than for the other frameworks, because the Si-O-Si angle usually differs from 180 . 

Derived 4 2 structures Si02 (tridymite), ice-I Si02 (keatite), ice-III... 

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