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    TEXT VERSION OF SLIDE:

    Title: Crystalline Silica
    Content:

    • SiO2 silicon dioxide
    • Also known as "free silica"
    • Significantly more hazardous than amorphous silica
    • 3 mineralogical forms
      - Quartzmost common
      - Cristobalite
      - Tridymite
    [Includes illustration of silica crystal and the OSHA logo]

    Speaker Notes:

    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.