Page 20 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 29,30
M i c a s

Micas rarely form more than a few percent of any
terrigenous sediment. even though they may be con­
�picuous in hand specimens. Muscovite is more common
than biotite since it is more resistant to weathering. The
source is usually a granitic or schistose rock. 29 and 30
show u typical micaceous sandstone. Note the parallel
alignment of the muscovite flakes and the concentration
at a particular level. This indicates the bedding. The thin
section has been positioned to show the second-order
interference colours of the muscovite. If the thin section
was shown with the bedding horizontal, the muscovite
Oakes would be at extinction.

29 and 30: Tifestones. Silurian. Uangadog. D.1fed. Wah•.>:
1/III.J.!IIi/icotioll x 16: 29 PPL. 30 X PL.
Other 111icas are sholl'n in 68 and 69.

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