Aluminosilicates species

We have recorded 388 minerals (Appendix 1) that occur, at least occasionally, as fibers some minerals are found only in fibrous form. This number includes more than 92 silicate and aluminosilicate species, most of them common rock-forming minerals. This list, only a fraction of the 3000 minerals known, probably represents only a sampling of naturally occurring fibers. 

It is proposed that in mixed organic base-alkali systems, the presence of the organic base changes the solid-liquid equilibrium and stabilizes larger sol-like aluminosilicate species ( 25 m/ ). The alkali ion affects agglomeration of the sol particles to larger amorphous precipitate particles from 100 to 500 min size which subsequently crystallize to zeolite. 

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

Mintova et al. studied deposition of zeolite A on various cellulose fibers pretreated chemically and/or mechanically [151]. It was shown that the amount of zeolite deposited was controllable by suitable fiber pretreatment with ball-milling or with diethyl ether under ultrasonic action. The reactive high-concentration hydroxyl groups on the structurally loosened celluloses seem to interact with aluminosilicate species and thus promote the formation of nuclei for zeolite crystallization. Pretreatment of natural cellulose fibers with alkali provides another simple route for anchoring preformed zeolite crystallites onto the cellulose surface. 

A typical zeolite synthesis involves mixing together silicate and aluminate solutions or sols to form an aluminosilicate gel, usually instantaneously, which is then treated hydrothermally to give the crystalline product. The composition and structure of the aluminosilicate gel are of considerable interest and characterization of the aluminosilicate species present would give insight into the crystallization process. 

NMR spectroscopy is ideally suited for characterizing the silicate and aluminosilicate species present in the media from which zeolites are formed. The nuclei observable include 29si, 27 Al, and all of the alkali metal cations. The largest amount of information has come from 29gi spectra ri-31. This nucleus has a spin of 1/2, no quadrupole moment, and a chemical shift range of about 60 ppm. As a consequence, it is possible to identify silicon atoms in specific chemical structures. 27 Al, on the other hand, is a spin 3/2 nucleus and has a sizeable quadrupole moment. This results in broad lines and limits the amount of information that can be extracted from 27Al spectra. 

There is a lot of evidence that the crystallization of zeolites from aluminosilicate gels is a solution-mediated transformation process in which the amorphous phase is a precursor for silicate, aluminate and/or aluminosilicate species needed for the growth of the crystalline phase (1-9). Generally, it is well known that the kinetics of most gel-zeolite and zeolite-zeolite transformations can be expressed mathematically by the simple kinetic equation (1,2, 10-12),... 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

The heating of the reaction mixture induces the growth of nuclei-I of both zeolites from the solution supersaturated with soluble aluminosilicate species. Since the growth rate of zeolite X is considerably greater than the growth rate of zeolite Na-Pc (see Figure 6), zeolite X appears as the first crystalline phase. The... 

It has been reported by F.A. Mumpton (1978) that more than 1000 occurrences of zeolite minerals in over 40 countries have been discovered since 1950. In addition to the hydrated aluminosilicate species, new minerals related to zeolites have been discovered, including the porous clathrasUs such as Melonophlogite (a silica only framework). Species in which the aluminum or silicon has been replaced by other elements (such as phosphorus, iron, and berylhum) have also been discovered as exemplified by viscite, a sihcoaluminophosphate related to analcime. At the present there are 3 8 different natural framework topologies, as shown in Table 25. 

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