Chemistry Reference
In-Depth Information
3.3.1.11 Pyrophyllite
Al
2
Si
4
O
10
(OH)
2
is a dioctahedral phyllosilicate formed by sheets similar to
those of muscovite (two tetrahedral layers (Si) surrounding an octahedral
layer (Al)). The sheets are linked by hydrogen bond; this very weak bond
explains the disordered stacking of the sheets, the good cleavage and the
low hardness of this mineral. Iron and manganese can be substituted in very
small amounts of aluminum.
This is a mineral of low temperature. Its stability is limited towards the
low temperatures (around 300°C) by the reaction:
H
2
O
and toward the higher temperatures (350-420°C) by
kaolinite
+
quartz
=
>
pyrophyllite
+
pyrophylite
=
>
quartz
+
alumina silicate
+
H
2
O
pyrophyllite
+
chlorite
=
>
chloritoid
+
quartz
+
H
2
O
pyrophyllite
+
calcite
=
>
margarite
+
quartz
+
H
2
O
+
CO
2
3.3.1.12
Diaspore (gibbsite and boehmite)
Diaspore
-AlO(OH) (with very low quantities of iron and manganese in sub-
stitution to aluminum) is formed of layers of oxygen in a hexagonal close pack-
ing; aluminum occupies 8-fold coordinated sites between the layers, forming
strips of octaedra.
Boehmite
α
-AlO(OH) is a polymorph of diaspore: double
sheet of octaedra with aluminium at their centers where the oxygens are in a
cubic packing relationship (instead of hexagonal close packing in diaspore). In
gibbsite
Al(OH)
3
the hydroxyls form sheets with hexagonal close packing.
Gibbsite and boehmite are mineral of supergene alteration. They are in par-
ticular, the major constituents of bauxites. Their very fine grain does not allow
their determination under the microscope. They will not be further dealt here.
Diaspore appears in the alterites (soils, bauxites). It is formed as soon as
diagenesis by recrystallization of gibbsite. It is a mineral of metamorphism of
the same peraluminous rocks. It destabilizes corundum to high temperatures.
β
3.3.2 Occurrences
3.3.2.1 Metamorphic rocks
Contact metamorphism
Contact metamorphism around igneous rocks, and particularly around,
granitoids, commonly forms spotted schists and hornflels with nodules of
cordierite and andalusite (often with the chiastolite habit). It is less common
that the metamorphism is strong enough so that sillimanite appears. Corun-
dum is very rare; kyanite and staurolite are exceptional.
