Page 23 - Atlas of Sedimentary Rocks Under The Microscope

33,34,35 Terrigenous clastic rocks
G l a u c o n ite

Glauconite is a hydrous potassium iron alumino-silicate
mineral which forms exclusively in marine environments,
usually in fairly shnllow waters. It commonly occurs as
rounded pellets which arc aggregates of many small
crystals. 33 nnd 34 show a number of glauconite pellets in
a coarse <;and•aonc. The glauconite is easily identifiable in
the photograph taken in plane-polarised light by its green
or brownish-green colour. The grain in upper centre part
of the field of view incorporates a number of silt-sized
quart7 grams. Glauconite has moderate birefringence.
but as the picture taken with crossed polars shows,
interference colours arc masked by the natural colour of
the mineral. The remainder of the sediment consists of
monocrystallinc quart..: grains and in the lower right
portion of the field of view. a sedimentary rock fragment.
The cement, which is showing high order interference
colours. is calcite.
35 shows a sandstone rich in glauconite and containing
�ubrounded quart..: grains (low relief) and carbonate
grains and cement (high relief). Note that many of the
bright green glauconite pellets have brown margins.
These are limonite and result from the oxidation of the
ferrous iron in glauconite.

•

33 and 34: !.ower Greensand. Lower Cretaceous.
Folkes10ne, J::nf(laml: magnification x 22; 33 PPL. 34
X PL.
35: !.ower Creraceous. Co. Antrim. Nortllem Ireland;
magnijimtio11 x 22. PPL.
Glauconite is shown also in2l4 and215.

17