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Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences

Published online by Cambridge University Press:  09 July 2018

P. Aagaard ,
J. S. Jahren ,
A. O. Harstad ,
O. Nilsen  and
M. Ramm
P. Aagaard*
Affiliation:
Department of Geology, University of Oslo, PO Box 1047 Blindern, 0316 Oslo
J. S. Jahren
Affiliation:
Department of Geology, University of Oslo, PO Box 1047 Blindern, 0316 Oslo
A. O. Harstad
Affiliation:
Department of Geology, University of Oslo, PO Box 1047 Blindern, 0316 Oslo
O. Nilsen
Affiliation:
Department of Chemistry, University of Oslo, PO Box 1033 Blindern, 0316 Oslo
M. Ramm
Affiliation:
Norsk Hydro Exploration, 1321 Stabekk, Norway
*
*E-mail: aagaard@geologi.uio.no
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Abstract

Grain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.

The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.

Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

Keywords

chloritesandstoneNorth Seaberthierine
Type
Research Article
Information
Clay Minerals , Volume 35 , Issue 1 , March 2000 , pp. 261 - 269
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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