Chemistry Reference
In-Depth Information
1.3.2.1 Parameters linked to the fluid phase
Parameters like fugacities (fH2O, fCO2, fS2 and fO2 ) belong to this; they
are particularly important factors of formation and evolution of the rocks
and their minerals.
For instance, the crystallization of amphibole, a hydrous mineral, or
of pyroxene, anhydrous mineral, depends on water fugacity and, thus the
evolution of a magmatic series is linked to the appearance of one or other of
these minerals. The silica content of pyroxene is about 50-55%, that of an
amphibole ranges from 40-45%. The silica content of the derived magmas
will be greater or smaller depending whether one or other of these minerals
appears.
Another example in a metamorphic domain is given by Buddington
(1963) who describes metric alternating beds of amphibolite facies rocks
(high water fugacity) and granulite facies rocks (very low water fugacity).
Oxygen fugacity controls the presence of graphite or the oxidation
grade of iron or manganese, and thus the appearance of such minerals as
hematite or magnetite or epidotes.
Fugacity of CO 2 controls the metamorphism of impure carbonate rocks:
according to whether it is high or not, the carbonates are preserved or trans-
formed into calcic or calcic and magnesian silicates.
Fluid inclusions in granulite facies rocks are rich in CO 2 (Touret, 1977,
1981) and sometimes are only made of CO 2 (granulites of southern India).
Various models have been presented to explain the role of such fluid in the
development of this facies: preferential dissolution of water in the magma or
supply of CO 2 “from a deep source” ....
A high sulfur fugacity favors the crystallization of iron sulfides, pyrite
and/or pyrrhotite, and thus, the iron contained in these sulfides do not par-
ticipate in the formation of the silicates.
1.3.2.2 Chemical composition of the rock
The rocks are classified into five major families:
1 The alterites and residual rocks , which are rocks where alkalis, calcium,
magnesium are more or less leached and are thus enriched in iron and
aluminum. These rocks are often rich in clay, illite and kaolinite. Bauxites
in particular, are made of iron and aluminum oxides and hydroxides:
goethite (FeO(OH)), limonites (FeO(OH), nH 2 O), gibbsite (Al(OH) 3 ),
diaspore (
α
AlO(OH)) and boehmite (
γ
AlO(OH)).
2
Sedimentary rocks are formed by deposition and diagenesis on the sur-
face of the Earth. They are classified into terrigenous clastic rocks and
chemical and biochemical rocks:
Clastic rocks may contain any mineral. In practice the nature of
the minerals reworked in sedimentary rocks depends on several factors:
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