Page 34 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 62, 63, 64
Greywackes

Greywackes are those sandstones containing more than
15°,'o fine-grained matrix. Their classification is shown in
Fig. C (see p. 24).
62 and 63 show a typical greywacke, being poorly­
sorted and containing abundant tine-grained matrix
(almol>t opaque in the view taken with plane-polarised
light). The fragments are predominantly monocrystalline
and polycrystalline quartz grains, but a small percentage
of rock fragments (cloudy particles of fine-grained
material) make this a lithic grcywacke.
64 and 65 illustrate a sediment with about 15% matrix
and contain1ng quartz and abundant f e ldspar grains.
Feldspars include plagioclase with multiple twinning and
perthitic alkali f e ldspar. The sediment is thus classified as
a fe ldspathic greywackc.
66 and 67 show a grcywacke in which quartz, f e ldspar
and rock fragments arc visible. Quart.£ grains are clear in
the PPL view. whereas the f eldspars arc broy, nish owing
to alteration. The XPL view shows that some feldspar
grains arc multiple-twinned plagioclase whereas others
are microcline showing typical cross-hatch twinning (e.g.
right of' field of view: about halfway up). The grain in the
centre of the field of view is an igneous rock fragment
consisting of plagioclase and amphibole. The amphibole
can be recognized by its green absorption colour and its
two cleavages at 1 2 0•. Smaller fine-grained rock frag­
ments and indi' idual f e rro-magnesian mineral grains are
also present.