Page 87 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Cherts 185, 186
C h e r ts

(continued)

185 and 186 show a silicified limestone in which silicific­
ation is not quite complete. The brownish grains seen in
PPL are unaltered calcite, as indicated by their high
birefringence when viewed with crossed polars. Those
grains which are clear in PPL show first-order inter­
ference colours when the polars arc crossed, and have
been entirely replaced by quartz. Although the limestone
has undergone considerable alteration, the ghost texture
visible shows that the original sediment comprised small
rounded grains (peloids) and a few shell fragments.

/85 and 186: AJ(e and locality unknown; maf(nific ­
ation x 22: 185 PPL. 186 X PL.

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