Page 97 - Atlas of Sedimentary Rocks Under The Microscope

210, 211, 212
P h o s p h a t i c sed i m ents

Some marine sediments contain authigenic phosphate,
usually in the form of a cryptocrystalline carbonate
hydroxyl fluorapatite, known as collophane. It commonly
occurs as ooids and pellets, or as biogenic material such as
fish teeth and scales or bone f r agments. Sedimentary
rocks rich in phosphate are called phosphorites.
210 and 211 show a phosphorite containing small
greyish-brown pellets of collophane set in a coarse calcite
cement. The view taken with polars crossed shows the
isotropic nature of collophane and the high-order inter­
ference colours of the calcite. Visible in the lower left­
hand quadrant is a grain of secondary quartz showing
first-order grey interference colours.
212 and 213 illustrate a phosphorite in which the grains
are principally brown-coloured pellets of isotropic collo­
phane. The colourless fragments, some of which show very
weak birefringence, are also phosphate. Some show a
trace of internal structure (e.g. the grain to the right and
below the centre). These are probably fish teeth and bone
fragments. In contrast to the phosphorite shown in 210
and 211 where the cement is calcite, the cement here is
fine-grained quartz.
214 and 215 show a limestone which has been stained
with Alizarin Red S and potassium ferricyanide (see p.
34). The fragments are mainly oysters (pink-stained) and
sections of hollow calcareous worm tubes (mauve) set in a
ferroan calcite cement (blue). The sediment contains
rounded pebbles of brown-coloured collophane (iso­
tropic), containing small quartz grains and scattered
pellets of green-coloured glauconite (see p. 17). The
sediment also contains a few large, rounded quartz grains
(e.g. the grain in the upper right corner).

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