Page 26 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 42,43,44
Cements

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42 shows a high magnification view of a fine sandstone
which contains both quartz and calcite cements. The
quart/ cement is in the form of overgrowths on detrital
grains. Evidence for this is the euhedral terminations seen
on �ome grains (good examples can be seen just above the
centre of the photograph). Unlike the quartz in the
sample in 38 and 39, the shape of the original detrital
particles is not visible where overgrowths are present.
Calcite cement postdates the quartz overgrowths and
infills pores. The thin section has been stained with
Alitarin RedS and potassium fc rricyanidc (sec p. 34) and
the calcite is a very pale mauve colour because it contains
some iron.
43 and 44 show a fine sandstone which is cemented by
gypsum. Gypsum has a pproximately the same birefrin­
gence as quart.�: and so it does not show up well in the XPL
photograph. In the PPL view. the higher relief of the
gypsum and its cleav<tge help to distinguish it from
quartz. Some of the gypsum cement crystals enclose
several detrital grains. One showing pale grey interference
colours in the XPL view occupies the upper left part oft he
photograph.

42: ivliddle Jurassic, Yorkshire. England: magnific­
o/ion x 72, PPL.
43 and 44: Cretaceous. Tunisia; magnification x 24; 43
PPL, 44 X PL.

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