Page 79 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 169, 170
D e d o l o m i t i z a t i o n

Dolomite may be replaced by calcite, usually by the action
of oxidizing meteoric waters. This process of dedolomitiz­
ation yields rhomb-shaped crystals of calcite or rhomb­
shaped areas which comprise a mosaic of replacement
calcite (dedolomite).
169 shows large rhomb-shaped areas which are now
pink-stained calcite crystals. The morphology of these
areas suggests that they were originally single dolomite
crystals. Note that the 'dedolomite' is full of brown
inclusions of iron oxides. This is a common feature since
dedolomitization often occurs in oxidizing conditions,
where any f e rrous iron in the precursor dolomite is
oxidiled to produce iron oxides rather than be in­
corporated in the replacement calcite. A sparry calcite
cement can be seen together with numerous hexagonal
sections through an unstained mineral with low relief,
which is authigenic quartz. Before dedolomitization this
sediment would have been similar to the calcitic dolomite
shown in 166.
170 illustrates a dedolomite in which the f o rmer
dolomite crystals have been replaced by a mosaic of small
calcite crystals. Micritic calcite sediment occurs between
the rhomb-shaped areas.

169: Stained thin section. Woo Dale Lime s tone. Lower
Carboni/'erous, Cunninl{ Dale, Derbyshire. En[{land: IIU/1(
n ( / ication x 27. PPL.
170: Swined thin section. U p p er Jurassic. Jebel Amsitten.
Morocco: lllllf{llijication x 42. PPL.
Dedolomite can also be seen in 137.

74