Page 77 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 163, 164, 165
D o l o m i t i z a t i o n

(continued)

163 shows a sediment in which the original limestone has
been totally replaced by dolomite. The result is a mosaic
of anhedral crystals. Although the section was immersed
in the staining solution, no stain at all is apparent,
indicating the completeness of the replacement.
164 shows a dolomite in which the crystals arc distinctly
loned. Although the crystal fa bric is tightly interlocking,
the rhombic shape of the dolomite crystals is clearly
outlined by the zoning. The zoning may be partly caused
by chemical dirlcrcnccs in the dolomite but it is probably
due mainly to varying amounts of foreign matter in­
corporated in the growing crystals.
The mineral dolomite may contain iron substituting for
magnesium. When the iron content reaches 10 mole %.
the term ankerite is used. 165 shows a ferroan dolomite
approaching ankerite.in composition. The f e rroan nature
of the mineral is �hown by the wrquoise stain colour (sec
p. 34), although the section was stained for a longer time
than usual to enhance the colour, which is why the calcite
present is red rather than pink-stained. The iron content
of the dolomite is also shown by the dark brown margins
to some crystals, where iron has been oxidized, producing
limonite.

163: Swined thin section, Pemnaen Burrows Limes/one,
Lower Carhon(ferous. Caswell Bay. South Wales: magnif ­
imtion x 43, PPL.
164: l.'nstained thin section, W o o Dale Dolomite, L01rer
Carboni e rous. Woo Dale. Derbyshire. England: nwgnific­
f
(l(ion x 56. PPL.
165: Stained thin section. W o o Dale Dolomite. Lower
Carhon{/erous. Woo Dale. Derbyshire. England; magnific ­
ation xJI. PPL.

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