Page 93 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Evaporites 200, 201
E v a p o r ites

(continued)

200 and 201 show an evaporite in which the minerals now
present arc halite (low relief. isotropic) and anhydrite
(moderate to high relief, second-order interf erence
colours). There is also some carbonate between the small
anhydrite crystals which is too fine-grained to be resolved
at the magnification shO\\ n here. The irregular six-sided
shapes which arc now composed principally of halite and
scattered anhydrite laths. have the form of gypsum
porphyroblasts (sec 195 and 196). They are thus interpre­
ted as gypsum crystals which have been replaced. The
gypsum itself was probably replacing anhydrite; this
Illustrates the complexity of diagenetic reactions which
may occur in evaporites.

20() and 20 / · Permian. II m,·,/., er Borehole. near ll'ltith r.
\ ortlt rork.\1111'(', England; magni{lcat ion X 8: 200 p Pl ..
201 XPL.

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