Page 51 - Sedimentology and Stratigraphy

38 Biogenic, Chemical and Volcanogenic Sediments

carbonates and bicarbonates of sodium and mag- ence of haematite causes the strong red colour of
nesium such as trona (Na 2 CO 3 .NaHCO 3 .2H 2 O), jasper, and traces of organic material result in grey
mirabilite (Na 2 SO 4 .10H 2 O) and epsomite or black chert. Thin-sections through chert reveal
(MgSO 4 .7H 2 O). All are relatively soft minerals and, characteristic patterns of either radiating fibres in
of course, all are soluble in water. chalcedony or completely interlocking microquartz
grains.

3.3 CHERTS
3.4 SEDIMENTARY PHOSPHATES
Cherts are fine-grained siliceous sedimentary rocks
made up of silt-sized interlocking quartz crystals Calcium phosphate occurs in igneous rocks as the
(microquartz) and chalcedony, a form of silica mineral apatite, which is a common accessory
which is made up of radiating fibres a few tens to mineral in many granitic rocks. Some apatite is pre-
hundreds of microns long. Beds of chert form either served in sediments as mineral grains, but gener-
as primary sediments or by diagenetic processes. ally phosphates occur in solution and are absorbed
On the floors of seas and lakes the siliceous skel- in the soil by plants or washed into the marine
etons of microscopic organisms may accumulate realm where it is taken up by plants and animals.
to form a siliceous ooze. These organisms are dia- Phosphorus is essential to lifeforms and is present
toms in lakes and these may also accumulate in in all living matter. Phosphatic material in the form
marine conditions, although Radiolaria are more of bone, teeth and fish scales occurs dispersed in many
commonly the main components of marine sili- clastic and biogenic sedimentary rocks, but higher
ceous oozes. Radiolarians are zooplankton (micro- concentrations are uncommon, being found most
scopic animals with a planktonic lifestyle) and frequently associated with shallow marine continen-
diatoms are phytoplankton (free-floating algae). tal shelf deposits. Most occurrences occur where
Upon consolidation these oozes form beds of chert. there is high organic productivity and low oxygen,
The opaline silica (opal is cryptocrystalline silica but not fully anoxic conditions. Rocks with concen-
with water in the mineral structure) of the diatoms trations of phosphate (5% to 35% P 2 O 5 ) are called
and radiolaria is metastable and recrystallises to chal- phosphorites (11.5.2). Mineralogically, phospho-
cedony or microquartz. Cherts formed from oozes are rites are composed of francolite, which is a calci-
often thin bedded with a layering caused by variations um phosphate (carbonate hydroxyl fluorapatite).
in the proportions of clay-sized material present. They In some cases the phosphate is in the form of
are most common in deep-ocean environments coprolites, which are the fossilised faeces of fish or
(16.5.1). animals.
Diagenetic cherts are formed by the replacement of Apatite is clear, with a high relief and is found quite
other material such as calcium carbonate by waters commonly as a heavy mineral in sandstones and may
rich in silica flowing through the rock. The source of be identified in thin-section. Biogenic phosphorites
the silica is mainly biogenic with the opaline silica of occur as nodules or laminated beds made up of clay
diatoms, radiolarian and siliceous sponges being to fine pebble-size material that is usually brown or
redistributed. Chert formed in this way occurs as occasionally black in colour. They can be difficult to
nodules within a rock, such as the dark flint nodules identify with certainty in the field, and in thin-section
that are common within the Cretaceous Chalk, and as the amorphous brown form of the phosphate may be
nodules and irregular layers within other limestones difficult to distinguish from carbonaceous material.
and mudstones. Chemical analysis is the most reliable test.
The dense internal structure of interlocking micro-
quartz grains and fibres makes chert the hardest sedi-
mentary rock. It breaks with a conchoidal fracture 3.5 SEDIMENTARY IRONSTONE
and can form fine shards when broken, a feature
which made this rock very significant in the develop- Iron is one of the most common elements on the
ment of tools by early humans. The colour is variable, planet, and is found in small to moderate amounts
depending on the proportions of impurities: the pres- in almost all deposits. Sedimentary rocks that contain