Page 82 - Atlas of Sedimentary Rocks Under The Microscope

173, 174 Other sedimentary rocks : Ironstones
I r o n s tones

(continued)

Chamosite readily oxidizes to limonite, sometimes on the
sea-floor soon after deposition, but more commonly,
much later after uplift and weathering. 173 shows an
ironstone containing both partially-altered chamosite
ooids, which arc yellow to golden-brown in colour with
darker limonite zones, and totally-replaced grains of pure
limonite which are almost opaque. The cement is calcite.
The sediment is fairly soft and some grains have been
plucked out during the making of the section, leaving
holes which appear colourless in the photograph.
174 illustrates an ironstone in which ellipsoidal grains
have been completely replaced by opaque limonite. The
sample contains a few rounded shell fragments (e.g. left­
hand side above centre) and scattered quartz grains,
which although colourless and showing low relief, arc
sometimes surrounded by thin rims of dark limonite
(examples can be seen near the centre). The cement is
calcite.

173: Northampton Sand Ironstone. Middle Jurassic.
Northwnpton1·hire. England: lllagnification x 37. PPI ..
174: Frodinglw111 lronswne. Lower Jurassic. Scunthorpe.
England; nwgn!frcation x 35, P PL.

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