Crystalline silicas classification

CAS 14464-46-1 EINECS/ELINCS 231-545-4 Synonyms Calcined diatomaceous earth Calcined diatomite Crystalline silica Crystobalite Melecristobalite Sand Silica, crystalline Silica, crystalline-cristobalite Classification Inorg. silicon compd. 

CAS 14808-60-7 EINECS/ELINCS 238-878-4 Synonyms Agate Amethyst Chalcedony Crystalline silica Flint Free crystalline silica Onyx Pure quartz Quartz glass Rose quartz Sand Sea sand Silica Silica, crystalline quartz Silica flour Silica glass Silica, quartz Silicic anhydride Silicon dioxide Classification Inorg. silicon compd. 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

Until 1985 silicas were usually divided into two groups natural and synthetic. Most of the naturally occurring SiC>2 modifications are crystalline, but this group also includes amorphous or mostly amorphous products, such as diatomaceous earth or kieselguhr. However, when the observations made at the end of the introduction are considered, this classification no longer proves sufficient, as fly ashes and silica fume are also synthetically produced, although not deliberately. These airborne dusts are not harmless (22). 

Silicates are by far the most abundant chemical groups of minerals. The crystalline structure is based on a tetrahedron with a central silicon ion attached to four oxygen ions. These silica tetrahedra may be polymerised in various ways, and this has led to the following universally applied classification of crystalline silicates in the mineral sciences. 

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