Chemistry Reference
In-Depth Information
below). Its stability is limited to high pressure values of around 16-17 kb.
Staurolite is rare at very low pressures. The paragenesis staurolite-cordier-
ite-biotite-muscovite is known in many places (although some authors deny
it for theoretical reasons).
3.3.1.4 Chloritoid
Chloritid is nesosilicates made of sheets parallel to the plane (001) (perfect
cleavage plane) connected by isolated (SiO
4
)
4−
tetrahedra. Each sheet con-
sists of a compact assemblage of octahedra. There are alternating layers of
Al
6
O
16
corundum-type layers and M
4
Al
2
O
4
(OH)
8
brucite-type layers. M can
be Fe
2+
, Mn, and Mg. In this brucite-type sheet, ferric iron can replace small
amounts of Al, the introduction of ferric iron is accompanied by an OH
deficiency. Hence the general formula of chloritoid is:
(Fe
2+
, Mg, Mn)
2
(Al, Fe
3+
)Al
3
O
2
(SiO
4
)
2
(OH)
4
The chloritoid is a metamorphic mineral of low to medium temperature,
stable at temperatures up to 600-650°C (first half of amphibolite facies).
Ottrelite
, manganoan equivalent of chloritoid, appears in areas of low
to very low levels metamorphic grade in manganese-rich levels (for instance
in the Ardennes). The percentage Mn/(Fe
Mn) can reach 60%.
A (very) low pressure (and low and medium temperatures), only the
iron chloritoid is stable. So this mineral appears only in the rocks of particu-
lar chemistry, both aluminum- and iron-rich and poor in alkali: for instance
metamorphosed lateritic paleosols or bauxites (and the sediments derived
from such paleoalterites).
At higher pressures the range of compositions of chloritoid widens: Mg/
(Mg
+
Mg
+
Mn) ratio can reach 40% and even, exceptionally, 74%. Chlo-
ritoid appears in more varied rocks. It is a common mineral in metapelites
of blueschist facies.
+
Fe
+
3.3.1.5 Cordierite
Classically, cordierite is considered as a cyclosilicate: (Si
6
O
18
)
12-
rings in
which part of the silicon atom is replaced by aluminum, connected by iso-
lated (SiO
4
)
4-
tetrahedra, where also a part of the silicon atoms is replaced by
aluminum. This structure of rings linked by tetrahedra, makes that cordierite
is sometimes considered a tectosilicate. The structure gives place to the octa-
hedral sites occupied by Mg and Fe
2+
. The common cordierites are mostly
magnesian, with a limited Mg Fe substitution.
There are two polymorphs of cordierite: the high temperature form,
indialite, is hexagonal and the lower temperature form is orthorhombic,
pseudohexagonal. The transition between the two forms is around 1450°C
