33. TRANSFORMATION OF OPALINE SILICA IN SEDIMENTS
X-ray powder diffraction analyses of sponge spicules and radiolarians taken from cores drilled at the Bay of Biscay and Rockall Bank in the northeast Atlantic show marked changes in silica minerals. In Hole 400A, opal-A exists in the Miocene-Oligocene sequence, opal-A coexists with clinoptilolite wh...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.569.2759 http://www.deepseadrilling.org/48/volume/dsdp48_33.pdf |
| Summary: | X-ray powder diffraction analyses of sponge spicules and radiolarians taken from cores drilled at the Bay of Biscay and Rockall Bank in the northeast Atlantic show marked changes in silica minerals. In Hole 400A, opal-A exists in the Miocene-Oligocene sequence, opal-A coexists with clinoptilolite which shows pseudomorph from biogenous shells in the Eocene-Paleocene sequence, and opal-CT coexists with the clinoptilolite in the Albian sequence. Formation of the clinoptilolite may be controlled by concentration of aluminum and alkali ions which is supported by silicate reactions observed in the interstitial water of the site. The mode of occurrence indicates that opal-CT alone occurs when calcium carbonate in sediments is over 80 per cent. The marked depletion of alkalinity in Site 406 and the scarcity of aluminum and metallic cations in interstitial solutions seems to favor precipitation of opal-CT. The origin of the bedded cherts in the Miocene-Oligocene and Eocene sequences at Hole 400A may be explained by the sedimentary processes such as turbidity currents and winnowing currents, for example, contour currents. A nodular type porcelanite at Site 406 may be formed by the deposition of silica as colloidal state which have been dissolved from the nearby diatomite layer. |
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