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Title: Crystalline Silica
Silica is a naturally-occurring mineral compound that occurs as both crystalline and non-crystalline (or amorphous) forms.
Crystalline silica is significantly more hazardous to workers.
Of the 3 crystalline forms, quartz is the most stable and is much more common. It is a major component of soils and is readily found in both sedimentary and igneous rocks.
--Granite is about 30% quartz.
--Shales contain about 20% quartz.
--Beach sand or sandstone may be nearly pure quartz.
Some volcanic rock contains cristobalite and tridymite. Heating (calcining) of diatomaceous earth (amorphous silica) converts it to a product containing up to 75% cristobalite. Cristobalite is also found in superficial layers of refractory brick subjected to contact with molten metal.