Incompatible Mineral Pairs in K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 281

1400

1300

Pl+L

Pl+Cor+L

1200

Lc +L
ss

Pl+Cor+Lc +L
ss

Pl+Cor?+L c +Mel+L
ss

P l+ Cor ? + L c +M el + D i+L
ss

1100
1080
Di

Pl+Cor?+Lc +Mel+Di +Ne +L

ss ss ss

Pl+Cor+Ne +Lc +Mel
ss

Pl+Cor +Lc +Mel+Di +Ne +L

ss

ss ss ss

25 30 35 45

+Ne +Lc

27 29 44

Wt.% An

Fig. 8.6 Phase diagram of the join (Di 27 Ne 29 Lc 44 ) 100-x An x at 1 atmosphere (after Gupta and Yagi
1980)

Gupta and Lidiak (1973) studied the system diopside nepheline leucite, and

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found that the final assemblage consisted of diopside ss , nepheline ss , leucite ss and

melilite. A mixture (Di 27 Ne 29 Lc 44 ), containing the same assemblage, was chosen as
one of the end members of a pseudobinary system to which anorthite was added to
study the incompatibility between the mineral pair melilite and plagioclase.
The phase diagram of the system (Di 27 Ne 29 Lc 44 ) 100-x -

An x is given in Fig. 8.6 ,

which shows the final assemblage in the system to be diopside ss + nephe-

line ss + leucite ss + plagioclase + corundum. Composition of melilite, as determined
from the mixture Di 27 Ne 29 Lc 44 at 1,100

°

C by the method of Hamilton and
MacKenzie (1960), is Ak 64 Sm 26 Geh 10 . The microprobe analysis of plagioclase
from a mixture of bulk composition (Di 27 Ne 29 Lc 44 ) 75 An 25 gave the composition
An 92 Ab 6 Or 2 . In leucite-bearing rocks, plagioclase is found to be calcium-rich
(Shand 1943; Savelli 1967), as found in this synthetic system. Schairer et al. (1965)
plotted the compositions of natural melilites in a composition triangle, where the
three end members were CaNaAlSi 2 O 7 ,Ca 2 Al 2 SiO 7 , and Ca 2 MgSi 2 O 7 . When the
composition of the melilite in the mixture of composition (Di 27 Ne 29 Lc 44 ) 75 An 25 is
plotted in this triangle, it falls in the field of natural melilite. Yoder and Schairer

(1969) studied the system akermanite anorthite at 1 atm and found that

plagioclase and melilite are compatible with liquid over much of the

albite

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field of pla-

gioclase, with the exception of compositions on the join akermanite albite. In a

diagram of ln a(SiO 2 ), versus temperature ( o C), Carmichael et al. (1970) plotted

-