Chemistry Reference
In-Depth Information
Neoformation
may also occur using ions (especially Mg) or col-
loids contained in the waters of the sedimentary basin, especially if it is
subjected to high evaporation which concentrates the ions and colloids.
Palygorskite and sepiolite are neoformed clays in conditions of evaporitic
environment.
Kaolinite, illite and more rarely mixed layer clays, may be authigenous
or formed during early diagenesis in sandstone pores. Kaolinite in particular
has an accordion-shaped habit due to the stacking of flattened pseudohex-
agonal crystals.
Glauconite forms in aerobic littoral environments, perhaps in associa-
tion with organic matter (foraminifera, pellets).
Diagenesis
due to burial is well documented by the study of oil drilling.
Diagenesis and anchimetamorphism produce:
a departure of the interlayer water (which may have a role in the migra-
tion of oil and gas);
a gradual recrystallization of illite;
mineralogical transformations: smectites become mixed layer clays and
then illite and chlorite, kaolinite is replaced by dickite and nacrite, and
later by chlorite and illite. Smectites begin to disappear at temperatures
of 70-95°C, that is, in the case of a normal geothermal gradient, at a
depth of 2-3 km. These changes suppose a supply of K
+
ions. The origin
of which is generally attributed to the destruction of clastic feldspars;
but, as many argilites or shales do not contain this mineral, perhaps the
origin of this potassium should be in connate waters.
Finally the only minerals that are stable under the conditions of anchimet-
amoprhism at the beginning of metamorphism are illite and chlorite.
3.6.2 Evaporites minerals
The principal minerals of this group are gypsum CaSO
4
· 2H
2
O, anhy-
drite CaSO
4
and halite NaCl. Compositions of these minerals are not very
variable.
The evaporites are rocks formed by chemical precipitation from seawa-
ter or continental waters.
Many evaporites formed by evaporation of seawater in shallow (par-
tially) isolated basins. The order of precipitation follows the order of
increasing solubility in seawater: calcite, dolomite, gypsum/anhydrite, hal-
ite, potassium and magnesium salts.
Calcium sulfates precipitate when about 30% of the initial seawater
remains. The normal form of calcium sulfate in subaqueous environment
is the hydrated form, gypsum CaSO
4
· 2H
2
O, that appears as well-formed
1-25 mm, often twinned, crystals, forming rosettes, stone roses, etc. In an
