Ternary Systems with Feldspathoids - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 259

Trachyte from Bufumbira,
Uganda (Cundari, 1975).
Leucite phonolite from
Bufumbira, Uganda (Cundari
and Le Maitre 1975).
Syenites (Nash and
Wilkinson, 1970).
Phonolite leucite tephrite
from Vico, Italy (Cundari and
Mattias, 1974).
Brown Leucitic Tuff with < 5.6
wt.% CaO (Luht and
Giannetti, 1987)

Leucite-bearing trachytic
pummice from Latera
Caldera (Turbeville, 1993)
Leucite Phonolite from Latera
Caldera (Turveville, 1993).
Albite-orthoclase minimum.
Brown Leucitic Tuff with > 5.6
wt.% CaO.

Qz

AB

KF

M

R

LC

Ne Ks

Fig. 7.3 Plot of bulk composition of Brown leucitic Tuffs (after Luhr and Giannetti 1987) in the
nepheline - kalsilite - SiO 2 ternary diagram, studied under 0.1 GPa [P(H 2 O) = P(Total)]

Albite

Orthoclase

60

40

Leucite field

70
765 0

30

Leucite

80 20

Nepheline field

90

10

V V V V V V

VV V

Nepheline 10 20 30 40 50 60 90

70 80 Kalsilite

Fig. 7.4 Phase relation in the system nepheline - kalsilite - SiO 2 under 0.2 GPa [P(H 2 O) = P(Total)]
(after Taylor and MacKenzie, 1975)

progressively eliminated. At 0.5 GPa, the field of leucite does not encompass

sanidine composition, and sanidine melts congruently. The Fig. 7.5 further shows
that the “ granite minimum ” becomes a eutectic. Likewise, “ the nepheline syenite

minimum has also become a eutectic. According to them there is a boundary

between the

”

fields of nepheline and kalsilite in the ternary system. This in turn