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Slide 5

    TEXT VERSION OF SLIDE:

    Title: What Is 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.

    Naturally occurring forms of amorphous silica include diatomaceous earth (the skeletal remains of marine organisms) and vitreous silica or volcanic glass.

    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.