Page 50 - Atlas of Sedimentary Rocks Under The Microscope

99, 100 Carbonate rocks
B i o c l a sts
Corals

Corals arc best identified by their overall morphology.
The rugose and tabulate Palacotoic corals were calcite.
thu� their microstructures are well-preserved. The walls
arc usually fibrous and small fragments which lack
evidence oft he characteristic coral form can be difficult to
1dcnufy.
99 shows a transverse section and parts of two longi­
tudinal <;cctions of the colonial rugose corn I U!hostrolion.
Note the thick outer wall and septa seen in the transverse
section. The columella and thin tabulae arc clearly visible
in the longitudinal section. Parts of' the coral walls have
been silicified (brownish colour). The pore-filling material
is mainly sparite cement with some micritic sediment
between the corallites.
100 shows a section through a tabulate coral. otc the
corallitc walls and thin tabulae but absence of other
internal structures. The infill is sparitc cement, initially
non-fcrroan calcite (pink-stained). but finally f e rroan
{blue-stained).
The Mcso.wic and Ccnowic sclcractinian corals arc
composed of aragonite and hence their microstructure is
not well-preserved in limestones. Sclcr<lCtinian corals arc
shown in 126, 144 and 145.

99· Swined 1hin sec1ion, Monml Dale Umeswne. Loll'er
Carboniferous. Coombs Dale. Derb1'.1hire. EnKiand: maK­
ni/IC ' fllion x 16. PPL.
100: Swined 1hin section. Torquay Umes1one. Devonian.
Brixl/(/111, Devon. EnKiand; maKni/icalion x 16. PPL.

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