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

Grain-coating chlorites in elastic quartz-rich sandstones have long been re cognized as an important porosity-preserving constituent in medium- to deep -burial diagenesis. As little is known about the occurrence and origin of c hlorite coatings, chlorite synthesis experiments were performed to study ho w grain-coating chlorites form in certain sandstones during burial. The sta rting material was naturally-occuring sandstones from the Oseberg and the V eslefrikk fields offshore Norway, where the same sandstone formation is bur ied to different depths due to faulting. Grain-coating chlorites exist belo w similar to 3000 m burial depth only. At shallower burial (2400 m), an X-r ay amorphous iron containing thin clay coating is present. The samples were heated to 200 and 250 degrees C (at water vapour pressure) in a hydrothermal bomb for 2-4 weeks. Both starting material and end-produ cts were studied (electron-) optically in both scanning and transmission mi croscopes. The TEM showed the Fe-rich precursor material to consist of a fi ne-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring ch lorite coatings. The TEM analyses of individual grains documented an Fe-ric h chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(A l0.58Si3.42)O-10(OH)(8). The reacted waters were found to be close to satur ation 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 temper ature around 90 degrees C.