Minerals framework

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

Most of the minerals which constitute rocks and soil are silicates. Many of these minerals have complex formulas, corresponding to the complex condensed silicic acids from which they aie derived. These minerals can be divided into three principal classes, the framework minerals (hard minerals similar in their properties to quartz), the layer minerals (such as mica), and the fibrous minerals (such as asbestos)... 

The Framework Minerals. A crystal of quartz may be desciibed as a single giant molecule, in trhich every atom is linked to the rest of the structure by silicon oxygen bonds. This crystal is hard and strong because any fracture requires that many of these chemical bonds be broken. The entire crystal may be described as made up of SiO tetra-hedra which are condensed together by sharing corners, that is, oxygen atoms, with one another the crystal has a tetrahedral frametvork structure. 

A characteristic feature of these tetrahedral framework minerals is that the number of oxygen atoms is just twice the sum of the numbei of aluminum and silicon atoms. In some of these minerals the frame... 

An interesting framework mineral is lazulite, or lapis lazuli, a mineral with a beautiful blue color. When ground into a po rder, this mineral constitutes the pigment called ultramariue. Lazulite has the formula Na8AUSio024(S ). It consists of an aluminosilicate framework in which... 

Write a general formula for an anhydrous sodium aluminosilicate which is a framework mineral, containing only tetrahedrally coordinated alu minum and silicon. 

We have seen that may replace Si". so long as electroneutrality is maintained by compensating cations. Three classes oF aluminosilicate framework minerals are of importance the feldspars, zeolites, and ultramarines. 

Qin S, Becerro AI, Seifert F, Gottsmann J, Jiang J (2000) Phase transitions in Cai cSr Ti03 perovskites effects of composition and temperature. J Mat Chem 10 1-8 Redfem SAT (1995) Relationship between order-disorder and elastic phase transitions in framework minerals. Phase Trans 55 139-154... 

Harrison RJ, Salje EKH (1994) X-ray diffraction study of the displacive phase transition in anorthoclase, grain-size effects and surface relaxations. Phys Chem Minerals 21 325-329 Hayward SA, Salje EKH, Chrosch J (1998) Local fluctuations in feldspar frameworks. Mineral Mag 62 639-645... 

Nearly 75% of all the minerals existing in the earth s crust are made up of the tectosilicates. At least nine different types are known to occur. Examples include both high (a) and low (/ ) quartz, as well as -cristobalite. The tectosilicates are themselves electrically neutral and do not need to be balanced by other types of cations. As a result of their interlocking framework, minerals of this type are very hard and have high melting points. Quartz, for example, is a seven on Moh s scale... 

A characteristic feature of these tetrahedral framework minerals is that the number of oxygen atoms is just twice the sum of the number of aluminum and silicon atoms. In some of these minerals the framework is an open one, through which corridors run that are sufficiently large to permit ions to move in and out. The zeolite minerals, used for softening water, are of this nature. As the hard water, containing Ca++ and Fe ions, passes around the grains of the mineral, these cations enter the mineral, replacing an equivalent number of sodium ions. 

MacGowan DB, Surdam RC, Ewing RE (1990) The effect of carboxylic acid anions on the stability of framework mineral grains in petroleum reservoirs. Soc Petrol Eng, Formation Evaluation, June, pp 161-166... 

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