Page 94 - Atlas of Sedimentary Rocks Under The Microscope

202, 203, 204 Other sedimentary rocks : Evaporites
Evap o r i tes

(continued)

Polyhalite, K2MgCaz(S04)4.2H20, is a common mineral
in some marine evaporite sequences. 202 shows a rock
composed essentially of polyhalite and halite. Both
minerals have a similar relief and crystals arc not easily
distinguishable in PPL. Thus only a view taken with
crossed polars is shown. The halite is isotropic and so
appears black. The polyhalite is partly fine-grained and
partly coarse-grained. The larger crystals show simple
twinning. Polyhalitc has a f a irly low birefringence, and
interference colours up to low second-order can be seen.
203 and 204 show a rock which is predominantly a fine­
grained polyhalite. Two large porphyroblast s of anhy­
drite with a typical lath shape can also be seen. The
distinct difference in relief of the two anhydrite crystals
results from their different orientations with respect to the
polarizer. The anhydrite crystal ncar the lower edge
shows the rectangular cleavage. The anhydrite is being
replaced by the polyhalite. The dark spots seen in 203 are
granules of bituminous carbonate.

202: Permian, Fort/on No. 1 Borehole, Scarborough. North
Yorkshire, England: magnification x 16, XPL.
203 and 204: Permian, Aislaby Borehole. near W h itby.
North Yorkshire . England: magnification x 20; 203 PPL,
204 X PL.

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