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The following observations were noted during fieldwork in Tethyan regions.
1. Radiolarians are often preserved where organic matter is abundant, generally in reduced environments or microenvironments resulting from transgression or confined basins. Such an anaerobic environment preserves silica from dissolution.
2. Radiolarian localities are commonly restricted to small basins (e.g., the Gulf of California).
3. In limestones, radiolarians are commonly restricted to small "nests" preserved in pyrite. This restriction may be a result of their sedimentation within fecal pellets, reducing microenvironments.
4. Preservation is generally better in rocks rich in clays (clayey limestones, nonglassy cherts). The clays may produce a double effect: (a) to form protective varnish all around the shell, (b) to slow the opal-A to opal-CT transformation so the structure of the opal-CT is better organized and much less subject to subsequent dissolution.
5. Radiolarians in limestones are generally calcitized. However, in rich limestones where silica is not concentrated in nodules but is "diffuse," radiolarians remain preserved in silica. This results from clays which limit (a) the opal-A to opal-CT transformation which occurs in a fluid state and (b) the migration of the silica fluid.
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