Chemistry Reference
In-Depth Information
T
°
C
P
H
2
O
= 3 Kb
= 5 Kb
P
H
2
O
1000
1000
melt
melt
800
800
FK + L
Ab + L
one mixed feldspar
FK
Ab
Solvus
600
600
two feldspar
Ab + FK
Ab + FK
400
400
NaAISi
3
O
8
KAISi
3
O
8
NaAISi
3
O
8
KAISi
3
O
8
Figure 3.6
Albite - potassium feldspar system (after Tuttle and Bowen, 1958).
a mixed sodi-potassic feldspar. This is what usually occurs in evolved vol-
canic rocks like phonolites, trachytes and many rhyolites.
The presence of water significantly lowers the crystallization tempera-
ture so that the liquidus and solidus intersect the solvus. This evolution is
called
subsolvus
crystallization. Then there is crystallization of two feld-
spars, a sodic feldspar (plagioclase) and a potassic one. Most granitoids
magmas show subsolvus evolution and crystallize two feldspars at the mag-
matic stage. Hypersolvus granitoids are very rare.
Ternary system anorthite - albite - K-feldspar
The crystallization of feldspar in magmas more calcic than granitic magmas
cannot be properly represented by the system albite - K-feldspar. So the full
ternary diagram albite - potassic feldspar - anorthite must be used.
This highly complex diagram must also represent the liquidus, solidus
and solvus. We will break it down here into:
A first diagram
showing the solidus and solvus under high water pres-
sure; the intersection of the solvus and solidus divides the field of feldspars
into two disjoint domains: plagioclase and alkali feldspar.
Under lower water pressure, the solidus is at a higher temperature and
intersects only partially the solvus: there is a continuous domain for a mixed
single feldspar near the anorthite-albite and albite-K-feldspar tie lines: this
is the case of hypersolvus crystallization.
A second diagram
shows the liquidus materialized by the isotherms: there
is a cotectic valley between the fields of plagioclase
alkali
feldspar. Temperature decreases along this cotectic line from the point E, the
An-FK tie line, toward the F eutectic point (close to the Ab-FK tie line).
+
liquid and liquid
+
