Chemical controls on the Opal-A to Opal-CT Transformation

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Since early Paleozoic the major mechanisms of silica precipitation at ordinary temperatures and pressures is biochemical. other mechanisms are: adsorption, organo-silicon complexing, evaporation or cooling of silica-rich waters (with subsequent precipitation), and neutralization of strongly alkaline solutions. Evidence from deep-sea sediments supports the following diagenetic sequence Opal-A (siliceous oozes) {yields} Opal-CT (porcelanite) {yields} chalcedony or cryptocrystalline quartz (chert) {yields} mega-quartz (chert). A solution and precipitation mechanism is involved in the above first two transformations. Exceptions to the overall maturation sequence are numerous, suggesting that temperature (burial depth) and time are not the only important factors that control the transformation of ... continued below

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Kastner, M. & Keene, J.B. January 1, 1976.

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Description

Since early Paleozoic the major mechanisms of silica precipitation at ordinary temperatures and pressures is biochemical. other mechanisms are: adsorption, organo-silicon complexing, evaporation or cooling of silica-rich waters (with subsequent precipitation), and neutralization of strongly alkaline solutions. Evidence from deep-sea sediments supports the following diagenetic sequence Opal-A (siliceous oozes) {yields} Opal-CT (porcelanite) {yields} chalcedony or cryptocrystalline quartz (chert) {yields} mega-quartz (chert). A solution and precipitation mechanism is involved in the above first two transformations. Exceptions to the overall maturation sequence are numerous, suggesting that temperature (burial depth) and time are not the only important factors that control the transformation of Opal-A to Opal-CT. The kinetics of the above transformations are strongly affected by the composition of the host sediments; in clayey sediments Opal-CT (porcelanite) predominates while in carbonate sediments quartz (chert) predominates. There is no simple way to relate either the crystalline state of silica or the texture of porcelanite and chert horizons to the age of surrounding sediments.

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  • Conference on scale management in geothermal energy development, San Diego, CA, August 2-4, 1976

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  • Report No.: COO-2607-4
  • Report No.: CONF-760844-14
  • Grant Number: E(11-1)-2607
  • Grant Number: NSF-G-AER-74-17445
  • Office of Scientific & Technical Information Report Number: 886869
  • Archival Resource Key: ark:/67531/metadc892466

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Office of Scientific & Technical Information Technical Reports

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  • January 1, 1976

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  • Sept. 23, 2016, 2:42 p.m.

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  • Nov. 22, 2016, 10:21 p.m.

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Kastner, M. & Keene, J.B. Chemical controls on the Opal-A to Opal-CT Transformation, article, January 1, 1976; United States. (digital.library.unt.edu/ark:/67531/metadc892466/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.