Geology Reference
In-Depth Information
Fig. 7.7 Phase relations in the pseudoternary system nepheline
-
kalsilite
-
SiO
2
determined at 2
GPa [P(H
2
O) = P(Total)] and various temperatures (present study). Plot of the ternary eutectic E3
is after Huang and Wyllie (1975). The symbol
“
▼
”
shows the compositions of the starting material
made to study the four binary joins Kls-Sa, Kls-Ne, Ne-Ab, Ab-Sa and the silica-undersaturated
portion of the system Ne-Kls-SiO
2
(Gupta et al. 2010)
Because of the reaction between nepheline and albite to produce jadeite at 2 GPa
in the presence of excess H
2
O, K-feldspar
jadeite becomes a thermal barrier.
Albite also breaks down to jadeite + quartz at 2 GPa [P (H
2
O) = P(Total)] in the
temperature range investigated. Thus, the albite
—
sanidine thermal barrier is no
longer valid at 2 GPa in the presence of excess H
2
O. Optical and EPMA study of
the compositions Ne
40
Kls
20
Qtz
40,
Ne
25
Kls
35
Qtz
40
and Ne
50
Kls
10
Qtz
40
show that
the subsolidus assemblage at P(H
2
O) = 2 GPa is alkali feldspar + jadeite
ss
+ quartz,
but in the same three compositions yield nepheline
ss
, leucite
ss,
alkali feldspar, liquid
and vapour below the solidus at low pressure, as albite-sanidine is a valid thermal
barrier under these P-T
—
T conditions.
The bulk composition of nepheline syenites from the Morro Redondo alkaline
complex, Brazil, has been plotted in the nepheline
-
kalsilite
silica system (curve
-
-
b
u in Fig.
7.9
). Different rock-types from the same petrographic province plot
along a curved path, as shown by the arrow. As the compositions of feldspars are
solid solutions, the trend of crystallization should follow a curved path, as was
obtained by Brotzu et al. (1989). If the tangents to the curve at various points are
-
Search WWH ::
Custom Search