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et al. (1976). These, together with the data of Washington (1917), have been plotted
in Fig. 7.9 , superimposed on the 2 GPa isobaric polythermal projection. One may
see that eutectic E 1 of the system nepheline
SiO 2 lies close to the maxi-
mum concentration of the shaded area of the compositional plot. It should be noted
that the eutectic E 1 related to the study of Gupta et al. (2010) is very close to the
minimum established by Hamilton and Mackenzie (1965).
Savelli
kalsilite
-
-
s (1967) compositions of potassic lavas from Mt. Vesuvius are plotted in
terms of nepheline
'
SiO 2 system (Fig. 7.9 ); they lie near the eutectic E 2 .
This observation suggests that although most of the feldspathoid-bearing mineral
phases crystallized near the surface, the lava might have originated at a depth of
60
kalsilite
-
-
65 km (2GPa).
Gittins et al. (1980) studied the potassic rocks of Batbjerg intrusion of east
Greenland in which nepheline and leucite are the important constituents. In these
rocks, they noted the occurrence of vermicular intergrowths of nepheline and K-
feldspar, and patchy to micrographic intergrowth of kalsilite and K-feldspar. The
assemblage near the eutectic E 2 comprises nepheline ss , kalsilite ss and K-feldspar ss
(Fig. 7.9 ). With the lowering of the temperature, initial composition of the liquid,
lies in the nepheline or feldspar
-
field, should move toward the cotectic M-E 2 with
co-precipitation of nepheline ss and alkali feldspar (the assemblage corresponds to
nepheline syenite). When the liquid composition reaches point E 2 , then simulta-
neous precipitation of nepheline ss , feldspar ss and kalsilite ss will take place. It may
be noted that the 2 GPa (equivalent to 60 to 65 km depth) eutectic E 2, plots in the
leucite ss
SiO 2 (Figs. 7.1 ,
7.2 , 7.3 , 7.4 , 7.5 , 7.6 ). Thus if a melt containing the assemblage nepheline ss
kalsilite ss liquid and vapour (E 2 , Fig. 7.8 ) ascend rapidly toward the surface, leu-
cite ss starts to precipitate in the intergranular spaces surrounding nepheline ss ,
kalsilite and sanidine phenocrysts, as leucite ss is a stable phase at low pressure. This
may be associated with concomitant resorption of the high-pressure phases. Relics
of the intergrowths of kalsilite
field of the low pressure isobaric diagrams of the Ne
Kls
-
-
K-feldspar and kalsilite-nepheline with leucite in the
interstitial spaces should still survive, as in the case of Batbjerg complex.
Kalsilite has been reported to occur in
-
fine-grained symplectites of a K-rich
phase including possibly sanidine, replacing leucite as a product of high
pressure
breakdown of the latter phase (Sandiford and Santosh 1991). This intergrowth of
sanidine and kalsilite replacing leucite is consistent with a high-pressure origin
(Scarfe et al. 1966). In the Synnyr alkaline pluton, northern Baikal (Zalutskii and
Chulkov 1971) and in lamproite dykes of Napoleon Bay, Baf
-
in Island (Hogarth
1997), a sanidine-kalsilite intergrowth (presumably in pseudoleucite) also has been
reported.
In the nepheline
SiO 2 system, the albite-sanidine join no longer acts as
a thermal barrier at 2 Gpa in the presence of excess H 2 O; instead, the jadeite-
sanidine join becomes the stable thermal divide. The compositions within the
triangle albite
-
kalsilite
-
jadeite yield an assemblage comprising quartz + sani-
dine ss + jadeite ss + liquid + vapour. At low pressure, the same compositions would
result in the crystallization of feldspathoid- and feldspar-bearing alkaline lavas. In
many petrographic provinces, such as Fort Portal and Birunga of Uganda
sanidine
 
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