Page 84 - Atlas of Sedimentary Rocks Under The Microscope

178, 179, 180 Other sedimentary rocks : Ironstones

I r onstones
(continued)

178 shows a limestone in which bioclasts have become
impregnated by opaque iron oxide. The iron oxide has
infilled the pores in crinoid fragments and partly replaced
their skeletons, leading to the development of a distinctive
reticulate structure. Bryozoans also have been impreg­
nated by iron oxide and two examples can be seen just
above and left of the centre of the photograph.
179 shows a thin section of a Precambrian banded
ironstone comprising alternations of dark, iron oxide-rich
layers and colourless chert layers. 180 is a higher magnif­
ication view of part oft he same section taken with crossed
polars and showing the fine-grained quartz which makes
up the chert layers.

178: Rhiu·bina Iron Ore. Lou'l!l ' Carbon(f'erous. South
Wales: magnification x 20. PPL.
179 and 180: Precambrian. T r a i /SI'aal: 179. magn(fic­
arion x 9. PPL: IBO. ma!tnification x 32. XPL.

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