Page 95 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Evaporites 205,206, 207
E v a p o rites

(continued)

205, 206 and 207 show a rock composed of anhydrite and
long narrow crystals of a carbonate mineral. The anhy­
drite occurs as separate rectangular crystals and as
sheaves of su b-parallel laths. One of these sheaves, having
first-order grey n terference colours, can be seen towards
i
the lower, right-hand corner of the photograph. Most of
the anhydrite crystals show two cleavages at 90•, and
bright second- and third-order interference colours. The
two photographs taken in PPL show the marked change
in relief of the carbonate mineral, achieved by rotating the
polarizer through 90•. 207, taken under crossed polars,
shows that this mineral has very high order interference
colours. It has been identified as magnesite, MgC03.

205, 206 and 207: Permian, Aislaby Borehole, near
Whitby, North Yorkshire, England,· magnification x 20,·
205 and 206 P P L, 207 X PL.

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