Page 22 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 31, 32
C h l o r ite

The sheet silicate mineral. chlorite, is abundant in sedi­
mentary rocks. It may occur as detrital flakes, usually
dcnvcd from low-grade metamorphic rocks, as an alter­
ation product. especially of volcanic rock fragments or as
an authigenic mineral filling pore-spaces. Plates 31 and 32
show a fine-grained sedimentary rock in which many
�mall fr<tgmcnt!> arc vi!>ible. but are less than l m m across
at th1' magnification. The rock is therefore a siltstone.
The larger rounded grains which are colourless in PPL.
and !>hO\\ ,lightly anomalous bluish-grey interference
colours m XPL. arc chlorite. In this case chlorite has
gro" n in the rock a!> a result of the breakdown of small
rocl.. fragments and fine-grained matrix during low-grade
mctamorphbm.

31 and 32 Ordol'ician. L/angranog, Dy(ed, Wales: '
1111/KIIi/icatioll x 72,31 PPL, 32 X PL.
Chlorite is shou•n also in 8. 58 and 59

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