Page 85 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Cherts 181, 182
C h e r ts

Cherts arc rocks composed of authigenic silica usually in
the form of fine-grained quartz. Cherts may be primary,
in which case most oft he silica is in the form of hard parts
of siliceous organisms such as radiolaria, diatoms and
some sponges. Much chert, however, is secondary, usu­
ally replacing limestone. Scattered grains of authigenic
replacement quartt, often hexagonal in cross-section, arc
not uncommon in limestones and are illustrated in 73 and
169.
Radiolaria arc siliceous microfossils which accumulate
in sediments of the deep ocean floor. 181 and 182 show a
radiolarian chert with spherical radiolaria skeletons and
their long thin spines. The matrix contains fine-grained
iron oxide. hence the red-brown colour. Radiolaria arc
originally opaline silica. an isotropic form containing.
water. This has been converted to fine-quartz (micro­
quart/) showing low first-order interf erence colours in
XPL.

181 o/1(/ 182: 1-rllt ' er Cretaceous. Greece: nwgnific·
ation x 32: 181 PPL. 182 XPL.
Calcite w.1ts l�/ ' radiolaria are shown in 89.

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