Chemistry Reference
In-Depth Information
for the pure magnesian end member; the presence of iron lowers this tem-
perature. Nevertheless this range of temperature is well above the tempera-
tures of geological phenomena: the common cordierites are orthorhombic.
In them, the Si/Al ratio hardly deviates from the value 5/4. The formula of
cordierite is:
(Mg, Fe)
2
Al
4
Si
5
O
18
The stacking of (Si, Al)
6
O
18
rings gives place to large channels that can
provide lodging for water molecules or other fluids. This water is not con-
nected to the lattice and should not be considered as a water of constitution,
but as zeolitic water. Nevertheless, water is always present in analyses of
cordierite. On the other hand, cordierite is most commonly altered into a
yellowish isotropic product called pinite, that is mostly chlorite and/or mus-
covite. It is difficult to analyze cordierite with the microprobe due to this
zeolitic water and such alterations.
The open structure of cordierite (tetrahedral sites, rings) means that this
mineral is unstable at high pressure: at the higher pressures, cordierite is
replaced by garnet
+
alumina silicate, or the association cordierite
+
K-feld-
spar is replaced by biotite (phlogopite)
alumina silicate. The upper limit of
stability of cordierite is around 6-7kb. Some authors consider much lower
pressures.
Cordierite is a mineral medium high temperature. Its melting point
(1450°C for the magnesian end member) lies beyond the field of geological
phenomena. At high temperatures, it may be replaced by associations of
spinel
+
+
quartz or sapphirine
+
quartz.
3.3.1.6 Magnesiocarpholite
Magnesiocarpholite is a fibrous inosilicate, the structure of which is quite sim-
ilar to that of orthopyroxene and whose formula is (Mg, Fe)Al
2
Si
2
O
6
(OH)
4
.
The iron end member is ferrocarpholite. The manganoan end member of the
series is carpholite, a rare mineral of pegmatites and metamorphic manga-
nese-rich levels. Carpholite was described as early as 1817; but it was not
until the use of the microprobe that magnesiocarpholite has been described
(Goffé et al., 1973).
The range of temperature of stability of magnesiocarpholite seems lim-
ited (350-600°C?). It is to replaced at lower pressures by chlorite:
magnesiocarpholite
+
pyrophyllite (Al
2
Si
4
O
10
(OH)
2
)
+
H
2
O
chlorite [(Mg, Fe)Al
4
Si
3
O
10
(OH)
8
)]
+
quartz
Magnesiocarpholite appears in metapelites in the high-pressure meta-
morphism with phengite-chloritoid-lawsonite-phyrophyllite. It seems char-
acteristic of the lower blueschist facies (with lawsonite, without jadeite).
