Pore-lining chlorites in siliciclastic reservoir sandstones: electron microprobe, SEM and XRD data, and implications for their origin

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Abstract
Pore-lining chlorites are often responsible for the preservation of porosity in deeply buried sandstones because they inhibit the formation of quartz overgrowths, but little is understood about when and how they form. Chemical analyses and XRD data indicate that there are at least two common types: Fe-rich, and Mg-rich. The Fe-rich examples occur as individual euhedral crystals, are invariably interstratified with 7 Å layers (probably berthierine) and form as the Ib β = 90° polytype at low temperatures. With increasing temperature, their chemistry changes, 7 Å layers are lost, crystal size increases, and eventually they transform to the high temperature IIb β = 97° polytype. The Mg-rich examples occur as boxwork arrangements of crystals, are not interstratified with 7 Å layers and are exclusively the IIb β = 97° polytype. The Fe-rich examples occur most frequently in sandstones that were deposited at the transition between marine and non-marine environments and the presence of Fe-rich oolites in many samples suggests a link to the ironstone facies. They probably formed originally at surface or near-surface conditions as a 7 Å mineral, such as berthierine or even odinite, in a fresh water/marine water mixing zone in tropical regions. The Mg-rich varieties tend to be found in aeolian or sabkha sandstones in close association with evaporites. They are probably replacements of Mg-rich smectites via the intermediate mineral corrensite. Precursor Mg-rich smectites formed originally from evaporite brines at near-surface conditions; chlorite itself was not formed until temperatures were high enough to crystallize the IIb β = 97° polytype.