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KAlSiO 4
(kal s i lite)
KAlSi 38
( san idine)
SiO 2
(quartz)
Sa
8
6
Ks
Q
9
2
18
5
Ph
16
3
1
4
En
15
R
11
19
14
Fo
10
13
7
phlogopite
Mg Si O
226
(enstatite)
Mg Si 2 4
(forsterite)
Fig. 12.13 Phase relations from the liquidus surface of forsterite- kalsilite-quartz at 2.8 GPa under
water-saturated condition. Numbered points are compositions used for experiments. The reduced
field of compositions fractionating to the silica-undersaturated minimum at 2.8 GPa in
consequence of both the expanded field of olivine crystallisation relative to enstatite and to the
role of phlogopite crystallisation (after Gupta and Green 1988)
within the forsterite
enstatite-sanidine
field fractionate toward the silica -undersat-
-
urated peritectic and eutectic points.
Under water-saturated conditions, the liquidus surface is at much lower temper-
atures and there is a large liquidus
enstatite cotectic
moves to more SiO 2 -rich conditions but this effect decreases as compositions move
away
field of phlogopite. The forsterite
-
from the
forsterite
quartz
join. The
peritectic
point R (forste-
-
rite + liquid
enstatite + phlogopite) lies at Fo 33 Ks 34 Qz 33 (i.e. within the forste-
rite
enstatite-sanidine compositional
field) and its temperature is 1160 + 20
°
C.
-
The thermal maximum along the forsterite
phlogopite reaction boundary lies close to
composition 3 (Fig. 12.13 ) and at a temperatures close to 1,200
-
°
C, the liquidus
surface of the phlogopite
C (Wendlandt and
Eggler 1980a, b) decreasing sharply to the eutectic (kalsilite + sanidine +
phlogopite) at approximately Fo 7 Ks 62 Qz 31 , T < 1075
field appears to be rather
at at
1,200
°
*
°
C. The peritectic
 
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