I can't answer the question, but I know cases of Neogene lake diatomite sediments where the diatoms are undamaged, perfectly preserved. These are of course composed of biogenic opal.
Major component of tests of radiolaria and diatoms and specific plant remains (silicified wood) are well preserved. If damaged it will certainly be the mechanical impact which counts. This is true for terrigenous as well as marine benthic parts. According to my experience only in case of strong changes in the physical-chemical regime a differential leaching may take place , e.g., hydrothermal solutions and pH changes towards stronger alkalinity. This may also happen when aging and deep burial take effect. In this case a recrystallization takes place irrespective of the origin (marine or lacustrine). Therefore, a general statement cannot be given without being familiar with the history of the rocks under consideration.
Opal exists as pure amorphous opal/opal-AG modifications of silica. They occur also in a variety of species as opal-C (disordered cristobalite) and opal-CT (disordered cristobalite and tridymite)
HGD
Maastrichtian- Danian opal CT - rich porcelanite displays largely fragmented sponge spicules locally changed to chert or silicous marl without ghosts of Opal CT
Hi Shahab Varkouhi,
I do not know the answer but I guess it depends on the depositional environment (marine vs. lake, coastal vs. open ocean, river delta vs. submarine channels...).
I have some questions according to your question: what is the benthic zone? when you mention biogenic opal, do you refer to diatom or radiolarian, or sponge spicules or...? what mechanical process can break down biogenic opal (I assume transportation by currents, either in the water column or at the sea floor, what else?what about )?
Thanks for posting the very interesting question!
По-видимоу, это зависит от генезиса (спикулы, спикульная решетка, панцири радиолярий) и размеров (ризоидные спикулы и т.д) элементов сложенных опалом. Кроме того, влияет и динамика придонного слоя водной среды: в зайстойной среде, на стадии диагенеза, в щелочной среде, может преобладать растворение, подвижность кремнезема. При динамичной среде - штормовой или ламинарной - механическое разрушение и, в последующем, растворение.
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