ABSTRACT

Mineralogy and relationships of the silica minerals were studied in 96 pelagic siliceous sediments from Legs I through VI of the Deep Sea Drilling Project. Heating cycles at 650°C and 1010°C were used to convert opaline silica to cristobalite, for purposes of estimating opal content. Most of the free silica in these sediments is biogenic (radiolarians and diatoms). Silica deposition was especially abundant during Eocene time. The experimental evidence confirms earlier hypotheses for the diagenetic sequence: solution of biogenic silica within the sediment, precipitation of poorly-ordered cristobalite, gradual inversion to microcrystalline quartz (chert). Cherts formed in clay, silt, and biogenic siliceous ooze all contain cristobalite, as do siliceous mudstones. Cristobalite is absent from the cherts in nannofossil chalk, and from soft biogenic siliceous oozes. New findings include: 1) silica in a calcite matrix may crystallize directly to quartz, without intermediate cristobalite; 2) certain biogenic amorphous siliceous oozes will yield synthetic cristobalite at the surprisingly low temperature of 650°C; 3) cristobalite formed by diagenetic processes is characterized by mixed-layer tridymitic structure, whereas cristobalite formed by heating truly amorphous silica lacks "tridymite" layers.