Transformation of Opaline Silica in Sediments from Bay of Biscay and Rockall Bank

Abstract

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.

Metallic Trace Elements in Some Chert Nodules of Pacific Seamounts: A Comparative Study

“…Although volcanic glass and the weathering of silicate minerals can contribute to the production of inorganic silica gel, the silica in nodular cherts is dominantly biogenic and derived from the dissolution of siliceous organic materials such as radiolarians, diatoms, and sponge spicules (Kagami, 1979). Biogenic siliceous particles are abundant in nodular cherts recovered from Shatsky Rise.…”

Silicified Sediments and Silica Diagenesis in the Goban Spur Area of the Northeast Atlantic, Leg 80

“…These studies have yielded many new insights regarding the occurrence and the nature of various siliciffed sediments. They have also revealed very diverse features of silica diagenesis and the formation of cherts and other siliceous sediments, such as porcellanites and diatomites (Heath, 1973;Garrison et al, 1975;Keene, 1975;Kelts, 1976;Kagami, 1979;Pisciotto, 1980;Riech, 1980;Grehin et al, 1981;Hein et al, 1981; Riech, 1981).…”

“…These studies have yielded many new insights regarding the occurrence and the nature of various siliciffed sediments. They have also revealed very diverse features of silica diagenesis and the formation of cherts and other siliceous sediments, such as porcellanites and diatomites (Heath, 1973;Garrison et al, 1975;Keene, 1975;Kelts, 1976;Kagami, 1979;Pisciotto, 1980;Riech, 1980;Grehin et al, 1981;Hein et al, 1981; Riech, 1981).Previous works have identified three main diagenetic silica mineral facies in these silicified sediments: (1) X-ray-amorphous silica (opal A), (2) disordered α-cristobalite (opal CT), and (3) stable quartz (Jones and Segnit, 1971). Also, silicified sediments can be classified into two major groups: a bedded type occurring in clayey or siliceous host sediments, and a nodular type found in predominantly calcareous host sediments (Heath and Moberly, 1971;Lancelot, 1973;von Rad et al, 1978; Riech and von Rad, 1979).…”