Silicates framework minerals

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)... 

With the exception of the mineral melanophlogite, all porosils are synthetic products. All porosils were directly synthesized with structure-directing agents (SDA) as templates, in general, organic molecules. There are a few more cases where postsynthesis treatment of alu-minosilicate zeolites leads to the complete dealumina-tion of silicate framework structures, yielding all-silica... 

Deer, W.A.,Howie, R.A., Wise, W.S. and Zussman, J. (2004). Rock-forrning rninerals. Volume 4B. Framework silicates Silica minerals, feldspathoids and the zeolites (2nd ed.). Geological Society of London, London. 

Hammonds, K.D., Dove, M.T., Giddy, A.P., Heine, V., and Winkler, B. (1995) Rigid unit phonon modes and structural phase transitions in framework silicates. Am. Mineral, 81,1057-1079. 

Aluminosilicates. These silicates consist of frameworks of silica and alumina tetrahedra linked at all corners to form three-dimensional networks familiar examples are the common rock-forming minerals quartz and feldspar. Framework silicates generally form blocky crystals, more isotropic... 

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

A variety of other structures are possible with silicate minerals, including sheets and three-dimensional frameworks. In all cases, the structure includes bonds that are predominantly covalent and directional. They can therefore be viewed as being based on increasingly crosslinked inorganic polymers. 

Finally, some authigenic clay minerals are produced by the reaction of seawater with fresh volcanic glass. This commonly occurs near mid-ocean ridges and rises or where lava from coastal volcanoes flows into the sea. Clay minerals produced by this process are primarily smectites, such as montmorillonite, and a type of framework silicate called zeolites of which phillipsite and clinoptilite are the most common marine examples. Zeolites are characterized by three-dimensional frameworks with large cavities that... 

Smith, J.V. and Bennett, J.M. (1981) Enumeration of 4-connected 3-dimensional nets and classification of framework silicates the infinite set of ABC-6 nets the Archimedean and. sigma.-related nets. Am. Mineral., 66 (7-8), 777-788. 

Kieflfer S. W. (1979c). Thermodynamics and lattice vibrations of minerals, 3 Lattice dynamics and an approximation for minerals with application to simple substances and framework silicates. Rev. Geophys. Space Phys., 17 35-59. 

When all four oxygens of the tetrahedra are shared (Si 0 = 1 2), a fully polymerized ion results, and a three-dimensional framework is formed. Quartz, an example of this type of silicate array, is discusssed in the section on silica minerals. 

The affinity between the tetrahedrally arranged orthophosphate oxyanion, P04, and hexava-lently coordinated metal cations lends itself to a classification of phosphate minerals in a scheme similar to silicates (SiO -) framework, insular, chain, and layer phosphates. Examples of this scheme, advanced by Povarennykh (1972) and further elaborated by Lindsay Vlek (1977), include berlinite (AIPO4 framework) hydroxyapatite (insular) monetite (CaHP04, chain) and vivianite (Fe4(P04)2-2H2O, layer). 

As one might expect there is an approximate correlation between the solid state structure and the physical properties of a particular silicate. For instance, cement contains discrete 8104 units and is soft and crumbly asbestos minerals contain double chains of 8104 units and are characteristically fibrous mica contains infinite layers of 8104 units, the weak bonding between the layers is easily broken, and micas show cleavage parallel to the layers and granite contains feldspars that are based on three-dimensional 8104 frameworks and are very hard. 

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