Chemistry Reference
In-Depth Information
Among muscovites, tschermakitic substitution Al
VI
Al
IV
(Fe,Mg) Si
leads from the above ideal formula to
phengite
. Generally low temperature
and high pressure muscovites are phengite, the ideal composition of musco-
vite is reached only for high temperature muscovites (close to the stability
limit of muscovite).
K Al
IV
Si substitution leads from muscovite to
sericite
and
illite
.
They are usually classified as clay minerals. There is a continuous series
between muscovite and illite.
Biotites are classified according to the two mains substitutions Fe
Mg
and Al
VI
Al
IV
Mg Si.
Titanium can enter the network of biotite by various substitutions. The
type of substitution actually occurring is closely related to geological proc-
esses reflected by the evolution of micas. In the presence of a titanium min-
eral like ilmenite acting as a buffer, the titanium content of biotite increases
with temperature, reaching 5 to 6 wt% under conditions of granulite facies.
These levels of titanium can be used as a geothermometer (calibrated for
pressures of 4-6 kb by Henry et al., 2005). Very little titanium can enter in
the network of muscovite.
3.2.1.2 Alterations of micas
Biotite is very alterable mineral and is commonly altered into chlorite at low
temperatures, in particular, in surface conditions. The titanium content in
the lattice then recrystallized as grains or fine needles of rutile, sometimes
showing the sagenite twinning.
Epidote and prehnite occur less commonly in elongated lenses in the
cleavages of biotite.
Muscovite is a hardly alterable mineral; it is common as clasts in clastic
sedimentary rocks.
3.2.1.3 Stability of micas
Illite is stable in the sedimentary environment and its gradual recrystalliza-
tion produces a mica of muscovite-phengite type. The proportion of phen-
gitic substitution in muscovite decreases as temperature increases, until a
composition of near ideal muscovite at the stability limit of muscovite.
The stability of muscovite is limited to high temperatures by the discon-
tinous reaction (Figure 3.11):
Muscovite
+
quartz
=>
K-feldspar
+
sillimanite
+
water
Phengites replace potassium feldspar at high pressure. In the presence
of potassium feldspar, phengitic substitution rates in muscovite can be used
as a barometer.
