After diatoms dissolve, they reprecipitate in the rock as various kinds of siliceous cement. This photomicrograph shows chalcedonic quartz (chalcedony) cement in fractured chert from the Monterey Formation. The chalcedony is actually clear to light brown, but this view was obtained through a special polarizing microscope which allows a sedimentary petrologist to observe the fibrous nature of the mineral.
Photo © Rick Behl (1997).
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