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

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 Mg 0.41 Fe 3.52 Mn 0.10 Al 1.51 (Al 0.58 Si 3.42 )O 10 (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.