Page 27 - Atlas of Sedimentary Rocks Under The Microscope

45,46 Terrigenous clastic rocks
C e m e nts
(continued)

45 and 46 show a quartz sandstone at high magnification.
Note the mica flake in the centre of the photograph. In the
field of view shown many of the intergranular pores are
unfilled (e.g. lower left) and arc thus black in the XPL
view. However, the quartt. grains and mica Aake in the
centre· of the v1cw arc surroUI)dcd by numerous small
crystals with low relief and showing first order grey
interference colour�. These are clay minerals in the form
of a cement. Usually an electron microscope is needed to
demonstrate the shapes of clay mineral crystals and
techniques such a� X-ray diffraction to determine the
exact identity of the minerals. In the example shown. the
crystab arc large enough for the typical low birefringence
of kaolinite to be seen, together with the 'book' texture
which develops as a result of the characteristic form of a
series of stacked platy crystals. This is best seen Im­
mediately above and to the right of the mica Aake.

4 5 and 46 Lmrer Carboniferous, F if eshire, Sc01lwul:
magnification >< 90; 45 PPL, 46 XPL.

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