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

Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark

^* E-mail: fj{at}geus.dk

(Received 29 May 1998; revised 29 January 1999)

In the Maastrichtian-Danian chalk in the North Sea, discrete intervals, appearing as normal white chalk, contain up to 60% -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 -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 -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: flocculation, nano-silica, North Sea