Page 62 - Atlas of Sedimentary Rocks Under The Microscope

131, 132, 133 Carbonate rocks

C a r b o nate e m e nts
c
(continued)

The characteristic texture of a cement precipitated in the
meteoric phreatic 7onc (below the water-table) is one in
which the crystals increase in size from the margins of
pores towards their centres. This is known as a drusy
mosaic and results from competitive growth of crystals
away from the substrate on which they nucleate. The
resulting f a bric is one of more or less equidimensional
crystals, sometimes known as 'blocky' or ·equant' sparitc.
131 illustrates a drusy mosaic in which cement crystals
show compositional zoning, the stain picking out changes
in the amount of iron in the calcite brought about by
changes in composition of the c irculating ground waters.
As explained on page 55, fc rroan calcite cements are
precipitated under reducing conditions. The zoning
mdicatcs the position of crystal faces during growth, and
shows that the crystals were cuhcdral at the time,
<tlthough growth to completely fill the pore spaces has led
to the final crystal shapes being anhedral. Crystal
boundaries f o rmed by crystals growing together in this
way arc known as compromise boundaries.
Where the component grains of a limestone are com­
posed of a few large crystals. it is often possible to see that
cements 'lave been precipitated in optical cominuity with
the grains on which they nucleate. These arc known <lS
.syntaxial o1·ergroll'ths or sy11taxial rim cementv and arc
most ea�ily seen on echinoderm fragments. 132 and 133
show a sediment in which the cement is composed entirely
of syntaxial overgrowths on crinoid plates. The crinoids
can be identified by their speckled appearance whereas
the cement is clear. The syntaxial nature of the cement is
shown in PPL by the cleavage passing through both
bioclast and cement. In XPL. the uniform extinction
colour of both crinoid and overgrowth can be seen.
Fragments of f e nestrate bryozoans are abundant in the
sample.

131: Stai11ed acewte peel. Woo Dale Umesttme. Loll'er
C a r boniferous, Wol/.lcote /)a/e. Staffordshire. England;
magnificatioll x 22. PPL.
132 and 133: Stai ned thin section. £yam Limestone. Loll'er
Carbonifemus, Once-a lt'l'ek Quarry. Derhphire,
England; magnification x 17. 132 PPI . 133 XPL.
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