Radiolarian chert

Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth. However, radiolarian earth is typically regarded by Earth scientists to be the unconsolidated equivalent of a radiolarite. A radiolarian chert is well-bedded, microcrystalline radiolarite that has a well-developed siliceous cement or groundmass.

Radiolarites are biogenic, marine, finely layered sedimentary rocks. The layers reveal an interchange of clastic mica grains, radiolarian tests, carbonates and organic pigments. Clay minerals are usually not abundant. Radiolarites deposited in relatively shallow depths can interleave with carbonate layers. Yet most often radiolarites are pelagic, deep water sediments.

Radiolarites are very brittle rocks and hard to split. They break conchoidally with sharp edges. During weathering they decompose into small, rectangular pieces. The colors range from light (whitish) to dark (black) via red, green and brown hues.

Radiolarites are composed mainly of radiolarian tests and their fragments. The skeletal material consists of amorphous silica (opal A). Radiolarians are marine, planktonic protists with an inner skeleton. Their sizes range from 0.1 to 0.5 millimeters. Amongst their major orders albaillellaria, ectinaria, the spherical spumellaria and the hood-shaped nassellaria can be distinguished.

According to Takahashi (1983) radiolarians stay for 2 to 6 weeks in the euphotic zone (productive surface layer to 200 meters water depth) before they start sinking. Their descent through 5000 meters of ocean water can take from two weeks to as long as 14 months.

As soon as the protist dies and starts decaying silica dissolution affects the skeleton. The dissolution of silica in the oceans parallels the temperature/depth curve and is most effective in the uppermost 750 meters of the water column, farther below it rapidly diminishes. Upon reaching the sediment/water interface the dissolution drastically increases again. Several centimeters below this interface the dissolution continues also within the sediment, but at a much reduced rate.

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