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Oolitic ironstone from Kilton Mine, Grid Ref. NZ 695169, Redcar & Cleveland, UK. The plane polarised light thin section shows ooliths. Ironstone ooliths often consist of berthierine or chamosite, both iron-rich chlorites. They are often found altered to goethite, an iron hydroxide, as is probably the case here. The lighter coloured spaces found in some of the ooliths are holes made during the thin section making process. |
The above photo shows the same thin section in cross polarised light. Both sections show that the groundmass consists of siderite an iron carbonate. The concentric structure of the ooliths is well displayed, illustrating how they were formed by oscillating currents leading to precipitation of chamosite around a nucleation point such as quartz or a fragment of chamosite itself, as can be seen in the oolith towards the top centre. Under cross polarised light, the extinction cross, caused by the concentric layering, is also apparent as can be clearly seen in the oolith slightly left of centre. |
