Page 92 - Atlas of Sedimentary Rocks Under The Microscope

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197, 198, 199 Other sedimentary rocks : Evaporites
Evaporites

(continued)

The two most common chloride minerals in evaporite
sequences arc halite (NaCI) and sylvite (KCl). 197 shows
these minerals together. The refractive index of halite is
close to that of the mounting medium, so that it shows
very low relief, whereas the sylvite has a moderate
negative relief. Some of the sylvite is reddish-brown in
colour owing to the presence of a small amount of
hematite. although the crystal in the lower right-hand
corner is hematite-free. The perfect { I 00} cleavage of
both minerals is visible in some crystals and the halite
shows some evidence of zoning. Both halite and sylvite
arc cubic and thus isotropic.
198 and 199 show a layered anhydrite-halite rock. The
thin layers of fine-grained anhydrite show moderate relief
in PPL and bright second-order interf e rence colours with
polars crossed. The halite has low relief and is isotropic.
The halite layers also contain scattered rectangular
anhydrite crystals.

197: Permian. Fison 's Borehole. Robin llood's Bay. North
Yorkshire. England: magnification x 20. PPL.
198 and /99: Permian. Fordon No. I Borelwle. Scar­
borough, Nonh Yorkshire. England: magnification x 9:
198 PPL. 199 XPL

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