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Precipitation and flocculation of spherical nano-silica in North Sea chalk

Published online by Cambridge University Press:  09 July 2018

F. Jakobsen ,
H. Lindgreen  and
N. Springer
F. Jakobsen*
Affiliation:
Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark
H. Lindgreen
Affiliation:
Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark
N. Springer
Affiliation:
Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark
*
*E-mail: fj@geus.dk
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Abstract

In the Maastrichtian-Danian chalk in the North Sea, discrete intervals, appearing as normal white chalk, contain up to 60% α-quartz <2 μm in size. Atomic force microscopy (AFM) reveals that the particles are of nm size, appearing as spherical particles and aggregates. Similar particles consisting of opal-CT were found in surface exposures of chalk in Denmark. Two new abiogenic pathways of silica formation in chalk are proposed. The first model proposes that SiO2 nano-size particles and aggregates precipitated and flocculated in the free-water phase as opal and were diagenetically transformed from opal-CT at low temperature to α-quartz at elevated temperature. In the second model, the dominance of radiolarians in the deep-water environment of the North Sea resulted in low dissolution supply with subsequent precipitation and flocculation of nano-size α-quartz particles. In the shallower water of the shelf environment of the present onshore chalk, the abundance of sponges and their dissolution supplied enough Si to precipitate opal-CT in the free-water phase.

Keywords

flocculationnano-silicaNorth Sea
Type
Research Article
Information
Clay Minerals , Volume 35 , Issue 1 , March 2000 , pp. 175 - 184
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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