Page 88 - Atlas of Sedimentary Rocks Under The Microscope

187, 188 Other sedimentary rocks : Cherts
C h e r ts

(continued)

187 and 188 show a variety of quartz types. The circular to
elliptical areas of fine quartz (microquartl) may be the
original sediment grains replaced by silica. The surround­
ing areas consist of clear and cloudy Lones of radial­
fibrous quartz. known as cha/cedonic quartz. The final
generation of brown silica illustrates most clearly the
radial-fibrous structure. Chalcedonic quartz is often a
pore-fill rather than a replacement. This is supported
from the evidence of the sample illustrated. in that there
are straight boundaries between adjacent growth of
chalcedonic quart.£ and triple points where three growths
meet. These polygonal boundaries arc characteristic of
radial-fibrous, pore-filling cements. The upper right oft he
photographs shows coarse equant quartz (macroquartz)
which contains inclusions of highly birefringent car­
bonate. indicating that the silica has probably replaced
limestone.

187 and 188: Upper Jurassic, Dorset. En�:land; magn({ic­
mion x 43; 187 PPL, 188 X PL.

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