Page 16 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 17. 18, 19
Rock f r a g ments

Rod f r agments. in particular metamorphic rock frag­
ment�. arc important contributors to many detrital
�edimcnts.
17 and 18 show a sediment with many rock fragments.
The two fragments in the centre of the photograph above
the large quart/. grain are made up of fine-grained
material which cannot be resolved at this magnification.
They arc fragments of shale or slate, and the character istic
platy shape is a result of derivation from a cleaved source
rock containing abundant platy minerals. The sediment is
very poorly-sorted. containing many small rock f r gmcnts,
quart/ grains and at least one twinned f e ldspar (in the
centre. ncar the top). as well as the large quartz grain, part
of which is seen at the base of the photograph.
Fragments of coarser-grained metamorphic rocks arc
often schistose. 19 and 20 show a fragment of muscovite­
bearing quartt-rich rock. The mica flakes show a prc­
tcrcntial alignment resulting in a schistose texture. Such
fragments arc sometimes classified as schisrose quarr:
rather than metamorphic rock fragments.
Sedimentary rock fragments, other than chert, arc
relatively uncommon in terrigenous sedimentary rocks
because they usually break down fairly easily into their
comronent grains. 21 and 22 show a large sandstone
fragment. Note that although the component particles
arc all quartz. they arc clearly distinguishable even with
PPL. This contrasts with the composite quart/. grain
\hnwn in 3 and 4, where individual crystals arc not visible
in PPL. The photograph taken with XPL shows that the
individual quartz grains are separated by a cement with
bright interference colours. This is likely to be clay.