Page 83 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Ironstones 175, 176, 177
I r o n s tones

(continued)

175 illustrates the abundant marine f a una found in many
oolitic ironstones. The speckled f r agments arc echinod­
erm plates (a large example can be seen just below the
centre). The concentric structure of the ooids is picked out
by alternations of green chamosite and opaque iron
oxides. The matrix contams iron oxides and small quartz
grains (colourless, low relief).
Ooids in Phanerozoic ironstones are often squashed
during compaction, suggesting that they were soft for
some time after deposition. This contrasts with calca­
reous ooids which are rigid grains and thus retain their
shape. 176 and 177 show chamosite ooids which have
been squashed and hooked as a result of compaction.
Such grains arc known as spastoliths. The very low
birefringence of the chamosite, with dark grey inter­
ference colours, is well seen in XPL. The matrix between
the ooids contains dark, greenish-brown coloured
chamosite mud and small roughly equidimensional
crystals with high birefringence. In some places these can
be seen to be rhombic in cross-section and are siderite
(iron carbonate). Larger brown-stained siderite crystals
can be seen replacing the margins of some of the ooids.

175: Loll'er Jurassic. Skye. Scotland; magnification x 16,
PPI ..
176 and 177: Raasay Ironstone, Lower Jurassic, Raasay.
Scotlmul; magn{{ication x 43; 176 PPL, 177 X PL.

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