Chemistry Reference
In-Depth Information
kaolinite: group
, structure of TO (1/1) type; the only cations are Si and
Al: kaolinite and its polymorphs (differing by the type of stacking of
the sheets), dickite and nacrite have no interlayer; halloysite contains
interlayer water;
illites group
, close to the micas of which they differ by a greater rich-
ness in silica and lower potassium content (substitution K Al
IV
Si ).
Sericite is a fine-grained muscovite/illite, product of alteration of pla-
gioclase. Glauconite is an illite containing ferric iron (ferrous iron and
magnesium) in substitution to octahedral aluminum;
smectites
form a large family of TOT (2/1) type structure which con-
tains Ca and Na in interlayer. Smectites are di-or trioctahedral. Among
the dioctahedral smectites, beidellites are purely aluminous, montmo-
rillonites contain magnesium in the octahedral site (and octahedral and
tetrahedral aluminum), nontronite contains ferric iron in octahedral site
(and aluminum in tetrahedral site). The principal trioctahedral smectite
is saponite which contains magnesium in octahedral site;
vermiculites
are trioctahedral minerals of TOT (2/1) type structure; the
interlayer is occupied by magnesium and to a lesser extent by calcium. The
octahedral site is occupied by magnesium, with aluminum and ferric iron.
mixed layer clays
(in French: “interstatifiés”
interbedded) made of
alternating sheets of more common minerals. Ordering of the sheets
may be regular or random. The most common are illite-smectite and
chorite-smectite;
=
fibrous clays of the
palygorskite group
are made of a stacking of ribbons of
TOT (2/1) type alternating with empty channels where zeolitic water may
enter: Palygorskite (or attapulgite) is aluminous, sepiolite is magnesian.
In common argillaceous rocks in the current size of kaolinite crystals
is about 5 microns, the illite crystals ranges from 0.1-0.3 microns, the
montmorillonite cystals even smaller:
clay minerals cannot be determined
under the petrographic microscope
but require the use of X-ray diffraction,
thermal analysis, electron microscopy, etc. However we include in the CD
a few clay minerals: glauconite recognizable by its color, kaolinite, which
in the pores of sandstones form characteristic accordions-shaped crystal of
relatively large size, etc.
3.6.1.2 Occurrences
Clays are
alteration
minerals:
hydrothermal
alteration, and on a much larger
scale,
pedogenic
alteration.
The rocks of the Earth's crust are basic (silicates, carbonates) and reducing
(ferrous iron, organic matter) media. Rainwater is oxidizing (presence of dis-
solved O
2
) and acid (dissociation of dissolved CO
2
: CO
2
+
H
2
O H
+
+
HCO
3
-
).
Alteration results from the desequilibium between these two media.
