Chemistry Reference
In-Depth Information
CaAl
2
Si
2
O
8
P0
P1
P2
P3
L0
L1
E
L2
S2
L3
S3
NaAISi
3
O
8
F
sanidine
KAISi
3
O
8
Figure 3.9
Crystallisation of the feldspars (after Best and Christiansen 2001, modified).
In projection on the plane albite - anorthite - k-feldspar (Figure 3.9),
the liquidus is represented by isotherms and its intersection with the solvus;
the shaded area represents the field of liquid
+
one single feldspar (under low
pressure water in the case shown).
Consider a magma that reaches the liquidus in L0 (Figure 3.8), this
magma crystallizes plagioclase P0, more calcic than the initial liquid
(Figure 3.9). Then during the cooling, the magma evolves along a line
L0-L1-L2, plagioclase by P0-P1-P2, and becomes more and more sodic.
The K-feldspar content of the liquid and of the plagioclase increases
until reaching the EF cotectic line in L2. There is then simultaneous crys-
tallization of P2 plagioclase and S2 K-feldspar. Then the liquid follows
cotectic line from L2-L3 crystallizing P2-P3 plagioclase that becomes
increasingly sodic (and potassic) and S2-S3 K-feldspar that becomes
increasingly sodic. In this simple case the magmatic plagioclase shows
a normal zoning from a more calcic center to a more sodic rim; the
K-feldspar shows a zonation with a more potassic center to a mixed
more sodic rim.
The cotectic line obviously stops at the boundary of the domain of sin-
gle feldspar. But crystallization may extend into this area until the minimum
temperature (“eutectic”) F point. In evolved volcanic rocks (phonolitic, tra-
chytes and rhyolites some) a single (mixed) feldspar then crystallizes: the
liquid has passed the final point of the cotectic line. In contrast, in plutonic
rocks that crystallized at lower temperature, the point F is located in two
