Non-crystalline, or amorphous, forms of silica exist when the silicon dioxide molecules are randomly arranged. Fly ash, silica fume and silica gel contain amorphous silica. Diatomaceous earth is 88% amorphous silica and is composed of the skeletons of small prehistoric aquatic plants related to algae [13]. Sometimes the compound may be hydrated, such as in opal.
Silica (both amorphous and crystalline) can transform. In unusual instances of extreme heat and very slow cooling conditions, amorphous silica can be transformed into crystalline silica as mentioned above. Similarly, various crystalline silica forms can transform into different crystalline forms when subjected to high heat or high heat and pressure. The presence of trace elements in the silica affects transformation rates. Heating rate is also important; on heating, cristobalite may form without first forming tridymite. Figure 2 illustrates some transitions between forms of silica upon heating and cooling.
Figure 2. Silica transitions with temperature[8].
3. HEALTH EFFECTS AND EXPOSURE STANDARDS
At least 1.7 million U.S. workers are potentially exposed to crystalline silica, which can cause the debilitating and incurable, but entirely preventable, lung disease called silicosis. Reported mortality associated with silicosis has declined to 200 to 300 reported deaths each year during the period 1992-1995. However, silicosis remains under-reported, perhaps because of a long latency between exposure and disease. In addition, an unknown number of unreported or undiagnosed worker deaths occur each year from silicosis and other silica-related diseases such as pulmonary tuberculosis (TB), lung cancer, and scleroderma. Occupational exposure to respirable crystalline silica is associated with the development of silicosis, lung cancer, pulmonary tuberculosis and airways diseases. These exposures may also be related to the development of autoimmune disorders, chronic renal disease and other adverse health effects [14].
