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annite
K 2 Fe 6 Si 6 Al 2 O 20 (OH) 4
Siderophyllite
K 2 Fe 5 AlSi 5 Al 3 O 20 (OH) 4
Biotites
Phlogopites
Phlogopite
K 2 Mg 6 Si 6 Al 2 O 20 (OH) 4
eastonite
K 2 Mg 5 AlSi 5 Al 3 O 20 (OH) 4
Figure 3.12 Biotites.
chlorite associations or stilpnomelane. Biotite appears towards the middle
part of the greenschist facies, that is at temperatures of about 400-500°C.
Towards higher temperatures biotite becomes unstable and is destabi-
lized by reactions similar the reactions to the disappearance of muscovite:
Biotite is not stable at low temperatures but is replaced by muscovite
+
Biotite
+
quartz
=>
orthopyroxene
+
feldspar
+
water
Aluminous biotite
+
quartz
=>
garnet
+
orthoclase
+
water
These are continuous reactions as the minerals involved contain both
iron and magnesium. The ferriferous terms are destabilized at temperatures
lower than the magnesian terms.
Stability of iron-bearing micas also depends on oxygen fugacity. Annite
is stable to higher temperatures as oxygen fugacity is lower. Phlogopite is
stable up to temperatures of about 900°C at low water pressure and at
about 1100°C at water pressures of 4-5 kb.
3.2.1.4
Occurrences of micas
Igneous rocks
In Bowen's reaction series, micas crystallize after pyroxene and amphib-
ole and more or less simultaneously with acid plagioclase and potassium
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