Page 89 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Evaporites 189, 190, 191
Evap o r i t es

Evaporites are rocks composed of minerals which pre­
cipitate from natural waters concentrated by evapor­
auon. Although only a few minerals are abundant in
marine evaporite deposits, complex textures may develop
as a result of the replacement of one mineral by another
during diagenesis. On evaporation of seawater, the first
minerals to precipitate after carbonate arc the calcium
sulphates. The hydrous f o rm, gypsum, (CaS04.2H20 )
occur� only ncar the earth's surface, whereas anhydri1e
(CaS04) is formed at the surface and also replaces
gypsum at depth.
189 and 190 shO\\ laths of gypsum partially filling a
ca\ it) in a dolomite rock. The dolomite shows the very
high relief and strong birefringence of a carbonate,
whereas the gypsum shows low relief and \\cak birefrin­
gence. The photograph taken with crossed polars shows
typical gypsum interference colours, up to first-order pale
grey.
191 and 192 show a thin section through a sediment
composed almost entirely of anhydrite. It can be distingu­
ished from gypsum by its higher relief and stronger
birefringence. In the example shown, the anhydrite is Ill
the form of laths with a radiating habit. The view taken
with crossed polars shows the bright second-order inter­
f e rence colours characteristic of anhydrite.
193 and 194 show a sediment composed of gypsum and
dolomite. The dolomite is very fine-grained and almost
opaque in the photograph. The gypsum is in t\\0 fonns.
At the ba�e and top oft he photograph it is in the form of a
network of irregular crystals, whereas in the centre It is 111
the form of fibres aligned at right angles to the bedding.
The f o rmer t)pC is characteristic of gypsum replacing
anh)dritc. \\hcrcas the fibrous gypsum is filling a vein
n1nning parallel to bcddmg.

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