Page 31 - Atlas of Sedimentary Rocks Under The Microscope

53,54,55 Terrigenous clastic rocks
Quartz a r e n i t e,

a r kose

53 shows a sandstone whi<.:h <.:onsists almost entirely of
quart1 and is thus clas�ificd as a quartz arenite. Such
sandstones were <.:ailed quart;itcs in older classifications.
although it is perhaps bcucr to restrict the term quartzite
to metamorphic rocks. Since they contain more than 95%
quartt. quart; arcntics are always mineralogically
mature. The example �hown is texturally submature to
mature. lackmg clay and being reasonably well-sorted.
Rounding of the grain� is difficult to assess because the
e ff e c ts of comract1on and cementation have obscured the
shape of the original grain!>.
54 and 55 �how a �cdimcnt in which more than 50% of
the grams arc f e ldspar. easily Identifiable in PPL by the
broY. n colour rcsuhmg from their alteration (see p. 7)
and in cros�cd polars by the remnants of multiple
\\\inning in many grains. Most of the other grains in the
sediment arc clear quart;. so the sample is an arkose. A
sediment with such a high proportion of relatively
unstable fe ldspar grains is mineralogically immature. The
matrix coni<! ins abundant opaque iron oxide.

53: Millstone Grit. Upp<'r Carhon(/<'rous. Craig-_r-/)inas.
Soutlt Wale.\: 11/Uf!lli/ication x 17. X P L
54 and 55: Torridonian. Premmhrian. 5)1'otlmlfl: magnific­
ation x 10:54 PPL. 55 X Pl-.

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