Page 60 - Atlas of Sedimentary Rocks Under The Microscope

126, 127 Carbonate rocks

c
Carbonate e m e n ts

The morphology and mineralogy of the pore-filling
cement crystals in a limestone can yield information
about the environment of cementation. Cements pre­
cipitated from marine pore-waters close to the sediment
-water interface may be aragonite or high magnesium
calcite, but in either case they may form small crystals
with a high length-to-width ratio. The crystals are aligned
at right angles to the surface on which the cement
nucleates. On curved surfaces this means that many
marine cements display a radial-fihrous fabric.
126 shows a section through a coral skeleton
(brownish-stained, structure not clearly visible) in which
the first generation of cement is acicular aragonite show­
ing a radial-fibrous texture. Note the variation in the
length of the crystals which gives a very irregular outer
margin to this generation of cement. Such a cement. being
aragonite. is not likely to be well-preserved in an ancient
limestone. If it undergoes neomorphism (p. 60), the
overall radial-fibrous fabric may be retained although
detatl will be lost. In the sample shown, there is a second
generation of pink-stained fine spa rite infilling pores. This
is typical of cement deposited from meteoric waters.
127 shows a limestone in which there arc also two
cement generations. The first appears as a rim of crystals
of equal thickness on all grains (about 2 mm in width in
th e photograph). Such cements are said to be isopachous.
The cement exhibits a radial-fibrous fabric although the
length-to-width ratios of the crystals arc not as great as
those in 120. 11 may originally have been aragonite, details
of the texture having been lost during inversion to calcite,
or tt may have been a high magnesium calcite marine
cement in which the crystals were elongate prisms rather
than needles. The final pore fill is an equant sparite, blue­
�taincd and thus f e rroan calcite. This latter cement is
characteristic of deposition from meteoric waters or from
connate waters fairly deep in the subsurface. In order to
incorporate fe rrous iron into the calcite lattice to produce
a f e r r oan calcite. reducing conditions must exist. If the
pore-waters arc oxidizing. any fe rrous iron present is
rapidly oxidized to ferric iron and precipitated as iron
h)droxide. Reducing conditions arc more ltkcly to occur
at depth than near the surface. Other coarse ferroan
calcttc cements are seen in 80, 87. 90 and 124.
116: Swined acetate peel. Quaternary. Momhasa. Kenya;
lllllf(!lijicarion x 70. PPL.
127. Stained acerate peel, Ommama11e F o rmation. Middle
Jurassic. Western High Atlas, Morocco: maKnific­
tllioll x 122, PPL.

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