Aluminosilicate polymorphs

Above 800 C an important chemical change takes place with the formation of one of the three aluminosilicate polymorphs (Al SiOj), i.e., andalusite, kyanite, or sillimanite, and free silica according to the overall chemical reaction ... 

From a plot of the saturation states of the silica polymorphs (Fig. 23.7), the fluid s equilibrium temperature with quartz is about 100 °C. Quartz, however, is commonly supersaturated in geothermal waters below about 150 °C and so can give erroneously high equilibrium temperatures when applied in geothermometry (Fournier, 1977). Chalcedony is in equilibrium with the fluid at about 76 °C, a temperature consistent with that suggested by the aluminosilicate minerals. 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

Trivalent transition metal ions substituting for Al3+ occur in several aluminosilicate minerals, including epidotes, yoderite and the Al2Si05 polymorphs andalusite, kyanite and sillimanite, often producing spectacular colours and pleochroism. As a result, numerous optical spectral measurements have been made of several natural and synthetic Al2Si05 minerals (Kerrick, 1990). 

In aluminosilicates, too, including epidote and the Al2SiOs polymorphs andalusite, kyanite and sillimanite, Al3+ ions occupy several coordination sites. Again, trivalent cations such as Cr3+, V3+, Mn3+ and Fe3+ may show cation ordering and be relatively enriched in one specific Al3+ site in a crystal structure. 

Aluminum is the third most abundant element in the lithosphere, but its levels in natural waters, plants, and animals are relatively low [1], It occurs in a vast variety of oxide minerals and together with silicon forms the polymorphous aluminosilicates (Al205Si) in rocks and soils upon which all organisms depend [2]. In soils and sediments these clays have many functions they hold trace elements, permit growth of roots, sustain pH balance, and their cavities keep water available [3],... 

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

Engelhardt, G., and R. Radeglia (1984). A semiempirical quantum chemical rationalization of the correlation between Si-O-Si angles and Si NMR chemical shifts in silica polymorphs and framework aluminosilicates. Chem. Phys. Lett. 108, 271-74. 

As a consequence of its composition, a zeolite framework may be charged or neutral. From the examples above, any silica polymorph and any aluminophosphate would be neutral. An aluminosilicate would carry a negative charge most zeolites have negative frameworks. In recent years, some zeolites have been synthesized whose frameworks carry a positive charge. 

Additionally, the secant relationship (4.20) established for silica polymorphs (Smith and Blackwell 1983) has been found to hold to within two ppm for a series of 12 layer-lattice aluminosilicates (MacKenzie etal. 1985) provided all T-O-T angles (including... 

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,... 

Several of the well-known polymorphic structural phase transitions that occur in minerals at low to moderate temperatures, particularly in framework aluminosilicates, have since been studied by NMR techniques a brief review of these studies is provided here. Also included is a review of NMR studies of quenchable cation ordering reactions, which further illustrate the types of information on mineral transformations available from NMR experiments. NMR spectroscopy has also been used extensively to study glass transitions and glass-to-liquid transitions, which have been recently reviewed by Stebbins (1995b). We begin with a brief description of NMR spectroscopy, focusing on those aspects that are most useful for studying transitions in minerals... 

The history of molecular sieve synthesis encompasses some 40 years and includes the preparation of aluminosilicate zeolites U), phosphorus-substituted aluminosilicates (2 3), and the microporous silica polymorphs (4). More recently, the compositional and... 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Limestone (chiefly calcite, CaCOa) and dolomite rocks (chiefly dolomite, CaMg(C03)2) are exposed at about 20% of Earth s surface. Carbonate detritus, fossil shell materials, and carbonate cements are also common in noncarbonate sedimentary rocks and arid-climate soils. The carbonate minerals found in such occurrences, in decreasing order of importance, are calcite, dolomite, magnesian cal-cites (Cai jMgfCOa where jc is usually <0.2), aragonite (a CaCOa polymorph) and, perhaps, magnesite. As a rule of thumb, when such materials are present in silicate or aluminosilicate rocks or soils at a level of about 1 % or more, they will lend to dominate the chemistry of the soil or ground-water. This fact is extremely important when one is concerned about the ability of a rock to neutralize acid mine waters, other acid wastewaters, or acid rain. 

In the model systems C through I (Table II) different sources of calcium and sodium were mixed with clay (kaolinite) and heated to observe their role in forming the aluminosilicates typical of ash fouling deposits. In systems C and D calcium acetate and sodium acetate were mixed with kaolinite in equal molar ratios. X-ray diffractometer patterns showed that both systems were, for the most part, amorphous at 750°C. However, in system E where sodium and calcium acetate were present in equal molar ratios carnegietite was formed at 750°C. Carnegietite is a polymorph of nepheline. In carnegietite the sodium cation is tetrahedrally coordinated whereas in nepheline the sodium cation is octahedrally coordinated. 

Quartz is present throughout, and is relatively unaffected. In the presence of quartz, kaolinite is the expected aluminosilicate phase. However, as mentioned above, gibbsite (Al(OH)3) or a gibbsite-like phase (such as one of its polymorphs, or an amorphous phase) is often found apparently to be controlling the dissolved A1 concentration. Very similar buffering stages would be observed in this case. 

Apart from the zeolites composed of aluminosilicates, there are also other families of microporous materials. One consists of aluminum phosphates which are AIPO4 polymorphs. These materials also exist in a wide range of open tetrahedral network structures. Structural modifications have also been carried out on these materials. An important example is the modification referred to as SAPO, in which silicon atoms replace some of the phosphorus atoms. 

Already by 1980 it was known that molecular sieve frameworks of compositions other than aluminosilicate could be prepared. Pure silica polymorphs of ZSM-5 and ZSM-11, for example, (patented as silicalite-1 and silicalite-2) were of interest because their internal surfaces were hydrophobic rather than... 

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