Geology Reference
In-Depth Information
Fig. 12.4 Phase diagram of
the system phlogopite
K
2
Mg
6
Al
2
Si
6
O
20
(OH)
4
. The
orthoenstatite (en)
—
high-p
clinoenstatite (cen) boundary
is from Pacalo and Gasparik
(1990) and Shinmei et al.
(1999) (both cited by Tronnes
2002). Cen clinoenstatite, En
enstatite, Fl fluid, Fo
forsterite, Ph phlogopite, Py
pyrope, X phase X.) (after
Tronnes 2002)
Phlogopite: K Mg Al Si O (OH)
2626 0
4
Py+X+Fo+Fl
±Cen
Py+Fo+Fl
10
Ph+Cen+Py
+Fo+X+Fl
Py+Fo+Fl
Phlogopite
stable
5
Synthetic ph
Synthesis exp.
Yoder & Kushiro (1969)
1000
1200
1400
o
Temperature ( C)
form of clinoenstatite and phase X are also present at 9
10 GPa. Tronnes also
showed that clinoenstatite reacts out relatively close to the breakdown curve and
disappears completely at lower pressure. Enstatite was not observed as a breakdown
product in the natural phlogopite system in the 4
-
8 GPa range by Sato et al. (1997,
cited by Tronnes 2002). Whereas Sato et al. (1997) noted an incipient breakdown of
the natural phlogopite to pyrope and
-
fluid at pressures of 5
-
8 GPa and temperatures
at 1200
C, the pure phlogopite end member remains stable in this P-T
interval. The amount of pyrope observed below the main phlogopite-out curve for
the natural phlogopite composition however, is very low ranging from less than 5
vol% at 5 GPa to 30 % at 8 GPa. This increase in, pyrope content with increasing
pressure is re
-
1300
°
ected in a systematic change in the composition of the coexisting
phlogopite.
12.3 The Join KAlSiO
4
-
Mg
2
SiO
4
-
SiO
2
up to 3.0 GPa
in Presence or Absence of H
2
O
Wendlandt and Eggler (1980a) studied the system forsterite
kalsilite-SiO
2
under
dry condition and in presence of CO
2
. They also took into account different other
reactions, involving various phases, compositions of which lie in this join. These
reactions include,
-
(1)
sanidine + leucite
liquid (Lindsley 1966), and
→
(2) 2 leucite
kalsilite + sanidine (Scarf et al. 1966).
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