Page 86 - Atlas of Sedimentary Rocks Under The Microscope

183, 184 Other sedimentary rocks : Cherts
C h e r ts

(continued)

183 and 184 illustrate f e atures found in limestones,
ironstones and terrigenous rocks as well as in cherts. The
sediment contains terrigenous quartz grains, both mono­
crystalline and polycrystallinc (p. 5), which arc clear in
PPL. It contains bioclasts, including an cndopunctatc
brachiopod shell (upper left) and echinoderm plates
impregnated with iron oxide (e.g. upper right). These are
composed of calcite and have been stained pale pink by
the use of Alizarin Red S (sec p. 34). The sediment also
contains silicified grains which arc unstained and brown­
ish in colour. These include both structurelcss examples,
which arc seen to be made up of microquartz in varying
orientations (e.g. below the centre), and those with a
nucleus of terrigenous quartz and an outer zone of silica
with an oolitic structure (e.g. left of centre, ncar the top).
These arc interpreted as silicified ooids. Evidence from the
XPL photograph suggests that in some of these at least.
the silica replacing the ooid cortex has grown syntaxially
with the terrigcnO\JS quartz nucleus (shown by uniform
mtcrference colour) e.g. the grain just below half-way up,
right of centre.

183 and 184: Stained tltin section. Carbonijc'1/1.1, Middle
Atlas, Central Morocco: magn{frcation x 17: 183 PPL. 184
YPL.

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