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Fig. 12.3 P-T diagram for
compositions in the join
K
2
O.6MgO.
Al
2
O
3
.6SiO
2
-
H
2
O. The solid
squares mark an invariant
point I and two singular
points II and III.
X = crystal + gas, circle with
an X = crystal + liquid + gas;
circle = liquid + gas. The
symbols are only relevant to
curves A and D. Curve A is
the maximum stability of
phlogopite in the presence of
a gas phase. Curve B marks
the beginning of melting of
the gas-absent assemblage
Ph + Fo + Ks (or Ok) + L and
curve C is the maximum
stability of phlogopite in the
absence of a gas phase. Black
dots are the data points of
Luth (1967) (after Yoder and
Kushiro 1969)
12.2.1 P-T Stability of Phlogopite up to 7 GPa in Presence
of Excess Water
Tronnes (2002) made experimental investigation on phase relations of pure
phlogopite and phlogopite peridotite. He observed that phlogopite broke down
around 7 GPa to potassium-richterite-bearing assemblages, which in turn, yielded
another assemblage comprising a hydrous phase containing high-K and low-Al.
The P-T stability of phlogopite has been determined by Tronnes (2002,
Fig.
12.4
), which shows that the decomposition curve is nearly coincident with that
of a natural phlogopite K
2.1
Na
0.1
Mg
5.4
Fe
0.2
Al
2.5
Si
5.7
O
20
(OH
1.4
F
0.6
), in the 4
8 GPa
range. The phlogopite breakdown curve of Yoder and Kushiro (1969) is shown for
reference in Fig.
12.4
. It may be noted that the high-pressure breakdown phases of
phlogopite is pyrope and forsterite +
-
fluid. Additional breakdown products in the
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