Page 81 - Atlas of Sedimentary Rocks Under The Microscope

Other sedimentary rocks : Ironstones 171, 172
I r o nstones

Scdimc111ary rocks with more than IS% iron, which have
usually been worked as orcs, arc known as irons/ones.
Phanerozoic ironstones arc usually local accumulations
of f o ssiliferous oolitic deposits and are called ooli1ic
irons/ones, whereas Precambrian ironstones are much
more extensive in area and comprise bedded <liternations
of iron minerals and silica. The latter arc known as banded
iron f o rmal ions.
171 and 172 show a Jurassic oolitic ironstone. The olive­
green mineral making up most of the ooids and the initial
cementing material is clwmosile. Chamosite is an iron
slltcatc with a structure similar to that of chlorite. It
occur� as fine-grained aggregates and, as the lower
photograph shows, it has a birefringence of nearly zero.
The brown-stained areas within the grains having high
btrcfringcnce arc siderile. iron carbonate, whereas the
mtcrgranular cement is calcite.

171 and 172· Nonlwmplon Sand Ironstone. M id d h •
Jurassic. i\'onhampton.111ire. Englallll: magnification x 37:
171 PPL. 172 XPL.

78