Silicates, Aluminates, and Phosphates

Chrysotile is a noncombustible fibrous solid that has been widely used as a fireproof thermal insulator, for brake linings, in construction materials, and for filters under the name of asbestos. It decomposes with loss of water at 600-800 °C, eventually forming forsterite and silica at 810-820 °C. Because it is more resistant to attack by alkalis than are the amphibole asbestoses, chrysotile has been used in chloralkali cell membranes and in admixture with Portland cement for making sewer pipes (Chapter 11). 

Asbestos fibers have been linked to the high incidence of lung cancer and mesothelioma (an otherwise rare pleural cancer) in asbestos workers and are considered to pose a threat to the population at large. Some 5000 deaths per annum in the United States have been attributed to mineral fiber contacts. There is now evidence that the true asbestoses such as amosite or crocidolite have a much higher association with mesothelioma than does the chrysotile used in most commercial asbestos, but controversy continues regarding the health hazards of asbestoses. The problem is evidently a general one with dust particles and fibers of any kind that are small enough 

Silica occurs widely in Nature as quartz, often in large transparent crystals of characteristic shape but also in the translucent agglomerations of microscopic crystals known as chalcedony, which includes cherts and flint. Other natural crystalline varieties of Si02 include tridymite and cristobalite (opal is a semiprecious stone that consists of microcrystalline, hydrous cristobalite). All forms of silica involve three-dimensional networks of corner-linked Si04 tetrahedra. 

If we were to replace some of the Si4+ ions with Al3+, it would still be possible to have a three-dimensional Si—0—A1 network, but cations would be needed to counterbalance the now anionic structural framework. Thus, for each substituent Al3+, we must add, say, an Na+, a K+, or half a Ca2+ ion. The feldspars, which along with quartz and micas (see below) are typical constituents of granites, can be viewed in this way  

K[AlSi308] orthoclase Na[AlSi308] albite Ca[Al2Si208] anorthite. 

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Nickel, copper, chromium, selenium and zinc hydroxides plus insolubles made up of silicates, aluminates, and phosphates of iron and calcium. 

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