Page 13 - Atlas of Sedimentary Rocks Under The Microscope

9, 10 Terrigenous clastic rocks
F e l d s p a r

Feldspars arc a major constituent of many sandstones
and conglomerates. Alkali feldspars are more common
than calcic plagioclase, partly because they arc more
resistant to chcmicul weathering. and partly because the
ultimate soun:e of many terrigenou� rocks is granite or
gneiss. rocks in which the f e ldspars arc mainly the alkali
varieties. The chemical weathering of feldspars may be
rapid, producing micas and clay minerals. Therefore
fe ldspars arc most abundant and best preserved in rocks
derived from mechanical weathering. The identification
of f e ldspars in thin section is straightforward in the case of
multiple-twinned grains of plagioclase or microcline. or
where pcrthitic textures arc present. Distinguishing be­
tween untwinned orthoclase and quartt can be difficult.
The fo llowing fe atures may help:
I. Alteration because orthoclase is more susceptible to
chemical weathering than quartl. it i� often cloudy or
brown-coloured in PPL. whereas quartz is usually
clear.
2. Refractive I ndcx the index of quartz is very close to.
but higher than that of Canada balsam. whereas the
index of orthoclase is always lower than balsam.
J. Interference llgure orthoclase is biaxial with a
moderate 2 V. quart;. is uniaxial unless strained.
9 and 10 show a large plagioclase grain which is easily
identified by the twinning in the photograph with polars
crossed. The grain �bows a combination of two types of
twins which arc probably Carlsbad (simple twin) and
albite (multiple twinning). The cloudiness seen in PPL is
caused h> patchy altcrat10n of the feldspar. The highly
birefringent. nne-grained alteration product is probably
sericite. a m•ca.

9 and /(}: Cohan Conglomera/e. Silurian. Rlwyader.
Wah's: nw�nificraion x 15: 9 ??L, 10 X PL.

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