Page 91 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Evaporites 195, 196
Ev p o r i t es
a
(continued)

Gypsum may replace anhydrite on uplift of evaporite
sequences and when removal of the overburden brings
them ncar the surface. Textures arc often of the type
hown in 193 and 194, with small irregular gypsum
crystals. but sometimes large cuhcdral crystals f o rm. 195
and 196 �how gypsum porphyroblasts replacing fine­
grained (aphanitic) anhydrite. Note the six-sided gypsum
crystals with low relief and first-order interference
colours. contrasting with the anhydrite showing
moderate relief and bright second-order interference
colours. Note that the distribution of relict anhydrite
inclusions within the gypsum porphyroblasts has some­
times led to the dcvclonment of a texture similar to "hour­
glass· LOning, a fe ature found in some minerals in igneous
and metamorphic rocks.

195 ond 196: Pl'rmitm. South Durham. Enr,/wu/: llttlf!11(fic ­
mion x 8 : 195 PPI • . /96 XPL.

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