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3.2.2 Chlorites
3.2.2.1 Structure and chemical composition
Chlorites are phyllosilicates formed from the superposition of talc-type lay-
ers similar to that of mica (two tetrahedral layers surrounding an octahedral
layer Y 6 Z 8 O 20 (OH) 4 ) and brucite-type layers formed by an octahedral layer
Y 6 (OH) 12 .(Figure 3.40).
The general formula of chlorite is:
Y 12 Z 8 O 20 (OH) 16
Y
=
Fe 2+ , Mg, Mn, Al, Fe 3+ , Cr 3+
Z
=
Si, Al
Chlorites vary mostly by the substitutions:
Fe 2+ Mg
Al VI Al IV
Mg Si
Al VI
<
=
>
Fe 3+
The non-aluminous magnesian end member Mg 12 Si 8 O 20 (OH) 8 , is not a
chlorite but antigorite (a serpentine group mineral). Manganese and chro-
mium are generally very minor constituents.
Chlorites are divided into (ordinary) chlorites and oxychlorites (or sep-
tochlorites) by the proportion of ferric iron. By convention, the boundary
between chlorite and oxychlorites is fixed to a content of 4 wt% of Fe 2 O 3 .
Each group is further divided into different terms according to iron/mag-
nesium and silicon/aluminum ratio (Hey's diagram, 1954, see Figure 3.16)
This nomenclature is not very useful.
Sudoite is a dioctahedral (thus rich in aluminum) magnesian chlorite, of
formula (Mg 4 Al 6 )(Si 6 Al 2 )O 20 (OH) 8 . It is stable only at (very) low tempera-
ture (anchimetamorphism, beginning of epizone).
3.2.2.2 Occurrences of chlorites
Chlorites are minerals rich in water and are not stable at high temperatures.
They occur in sedimentary rocks and low to medium grade metamorphism.
Chlorites are not of primary minerals of igneous rocks.
Sedimentary rocks
Chlorites are important constituents of the sedimentary rocks as clastic or
authigenic minerals. They can also come from the diagenetic recrystalliza-
tion or anchimetamorphism of clays of the smectite group.
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