Page 40 - Atlas of Sedimentary Rocks Under The Microscope

72, 73, 74 Carbonate rocks

O o i d s

Ooids o r ooliths arc spherical o r ellipsoidal grains, less
than 2 mm in diameter, having regular concentric laminae
developed around a nucleus. Ancient ooids often show
both the concentric laminae and a radial structure. It is
not always certain whether the radial structure is primary.
or fo rmed during the inversion of aragonite to calcite.
72 shows ooids with well-developed radial and con­
centric structures. The nuclei arc micritic carbonate
grains. The sample shows a range of ooids. from those
with a small nucleus and thick cortex (the oolitic coating).
to those with a large nucleus and a single oolitic lamina.
The latter are called superficial ooids. The matrix between
the ooids is a mixture of carbonate mud and sparry calcite
cement.
73 illustrates ooids with a rather poorly-preserved
concentric structure. The structure may have been partly
lost by micritization (p. 54). The speckled plates with thin
micrite coatings are echinoderms (an example can be seen
half way up the right-hand edge). The pink-stained
cement is non-ferroan sparry calcite. The unstained
grains with low relief are secondary (authigenic) quartz
replacing calcite.
74 shows ooids with relatively thin cortices coating
detrital quartz nuclei. Note how the early ooid laminae fill
in depressions on the surface of quartz grains and arc
absent from angular corners. The cement is pink-stained
non-ferroan sparry calcite.

72: Stained thin section, Upper Jurassic, C a p Rhir,
Morocco; magnification x 31. PPL.
73: Stained thin section. 1/unt 's B a y Oolite. Lower Carbo­
niferous, South Wales: magnification x 43. PPL.
7tl: Stained thin section, Carboniferous Limestone. Llan­
gollen, Clll'yd. Wales; nwgnification x 27, PPL.
Ooids can also be seen in 125, 127. 137, 146. 147 and 155.

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