Page 14 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks II, 12, 13
Fel d s p a r

(continued)

II and 12 �how a p..:bble-si7ed rragrnent composed almost
entirely or m•croclinc. Microclinc can be identified easily
hy the cross-hatched twinning which it invariably shows.
Although the microclinc shows little alteration. feldsp<�r
grams in the upper left. including multiple-t,�mned
plagiocla�e. arc browni!>h coloured as a result of alter­
all�m. ln contrast. the quartz in the upper right is
rclat•,·cl� clear and unaltered.
Graul!> �hO\\ ing perthitic intergrO\\ ths. comprising
hlchs or lamellae of sodium-rich feldspar in potassium­
rich feldspar. arc not uncommon in sediments. 13 and 14
�hO\\ a very coarse sand-sized fragment of perthite. Most
of the other sediment grains are quartz and the matrix
contains highly birefringent mineral grains too small to
identify at the magnification shown.
15 and 16 show grains of orthoclase and quart;. 1 he
feldspar can he identified in the PPL view by ib cloudy
appearance due to alteration. The quart7 is clear and
unalt..:rcd. ln the photograph taken with polars cross..:d. it
can he �cen that one feldspar exhibits a simple Carlsbad
twin (upper right of photograph). but most of the grains
ar..: not twinned. Two multiple-twinned plagioclase
cry�tals arc also visible in the lield of view.

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