Page 10 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks I, 2

Qua rtz

The most abundant grain type in sandstones and con­
glomerates is quarll. In addition to the size and shape of
individual quart£ grains. the following features should be
ob�erv..:d since they may provide clues to the provenance
of the sediment:
I. Whether the quartz grains arc single crystals (mono­
cry stalltnc) or arc made up of a number of cry stab an
diiTerent orientation!> (polycrystallinc).
2 Whether extinction is uniform (the grain extingui�hc�
in one position on rotation of the stage) or undulose
(the grain cxtingui�hcs O\'cr a range of at least s· on
rotation of the stage).
3. The presence or ab�ence of inclusions.
4. In the ea�c of polycrystallinc grains. whether the
crystal boundaries arc straight or sutured.
1 and 2 �how subroundcd quartz grains which arc single
cryst;ds. taken with plane-polarized light (PPL) and with
crossed polars (XPL). The matrix between the sand grains
contains opaque iron oxide and some calcite. The Iauer
shows high-order pink and green interference colours.

I {{//(/ 2: Red M ouJIIain l·imnarirm. Silurian. Birmin�f111111.
!flahr111w. USA: nw�nification x38; I PPL. 2 X Pl..

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