Geology Reference
In-Depth Information
Chapter 7
Ternary Systems with Feldspathoids
In the fractionation scheme of basaltic magmas, CaO, MgO and total iron oxide
(FeO + Fe
2
O
3
) contents decrease progressively, and those of Al
2
O
3
,Na
2
O and
K
2
O increase with respect to silica (Harker 1909), as a result of which the residual
liquid becomes enriched with respect to NaAlSiO
4
(nepheline), KAlSiO
4
(kalsi-
lite), and SiO
2
components. Bowen (1928), therefore, referred to the system
NaAlSiO
4
-
KAISIO
4
-
SiO
2
as petrogny
'
s residua system, in which the NaAISi
3
O
8
(albite)
KAlSi
3
O
8
(K-feldspar) join acts as a thermal barrier at low pressures. The
alkali feldspar join thus divides the system into a silica-de
-
cient and a silica-
saturated portion. The albite-K-feldspar-SiO
2
has relevance to the rocks of granite
family, and the silica-undersaturated portion has bearing on the genesis of felds-
pathoid-bearing ijolites, nepheline syenites ultrapotassic rocks and pseudoleucite.
7.1 The System Nepheline
Kalsilite
SiO
2
Under Variable
-
-
P
T Conditions at or Below 5 Kb in Presence of Excess
Water
-
Schairer and Bowen (1935) and Schairer (1950, Fig.
7.1
) studied this system under
atmospheric pressure. They established extensive solid solution relationship
between albite and orthoclase under one atmosphere, and the join does not have any
eutectic, but has a temperature minimum; hence in the ternary join albite-ortho-
clase-SiO
2
, there is a cotectic line F
H and instead of a eutectic, there is also a
thermal minimum (point G, Fig.
7.1
). This minimum is also popularly known as the
“
-
. With the increase of pressure in presence of H
2
O (up to 0.3
GPa), the granite minimum shifts toward albite (Tuttle and Bowen 1958). Slightly
above 0.35 GPa, the albite-orthoclase join has a eutectic and the study of Morse
(1969) showed the presence of a eutectic in this join at Or
28
Ab
72
under 0.5 GPa.
Since sanidinemelts incongruently at high temperature to formleucite and a silica-
saturated liquid, there is a
granite minimum
”
field of leucite, even in the silica-saturated side of the
albite-K-feldspar join. Since this volume is related to the genesis of silica-
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