Page 29 - Atlas of Sedimentary Rocks Under The Microscope

so, 51, 52 Terrigenous clastic rocks
G r a i n s o l u t i o n a n d

repl a c e m e n t

5 0 shows a porous sandstone. In this example the
mounting medium has been impregnated with a dye so
that the pores appear mauve-coloured. Note that the
margins of some of the quartz grains arc cmbayed. This
has occurred as a result of corrosion of the quartz during
diagenesis and has led to enhancement of the porosity.
The common porosity types in sediments are illustrated in
Fig. F (sec p. 65) and in 151-160, with examples from
limestones. Many of the terms are equally applicable to
sandstones.
51 and 52 show a sandstone cemented by a f e w large
calcite crystals. Note the typical high-order interference
colours displayed by the calcite. seen in the XPL photo­
graph. The detrital grains, including both monocry­
stallinc and polycrystalline quartz, arc coated with a thin
brown rim of iron oxide. The texture of the rock is
unusual, in that it is apparently not grain-supported.
being about 30% quartz grains and 70% calcite. Replace­
ment of original detrital grains by calcite is partly
responsible for this appearance, areas of calcite outlined
by iron oxide being interpreted as the original grains. A
g o o d example can be seen in the centre.

50: Saltll'ick Formation , Middle Jurassic, Eskdale.
Yorkshire. England; magnification x 132, PPL.
5/ all(l 52: New Red Sandstone, Tria�·sic, Bri.r:lwm, Devon.
En�rla11d; magnification x 43; 51 PPL, 52 X PL.

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