Ternary Systems with Feldspathoids - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 260

SiO 2

0

1065 C

Silica

P = 5 b
H 2 0

NaAlSi 38

KAlSi 38

0

758 ± 3 C

876 0

7 00
68 0
660

KAlSi 26

Leucite ss

Nepheline ss

Kalsilite ss

V V V V V V V V V

0

880 C

NaAlSiO 4 KAlSiO 4

Fig. 7.5 The system nepheline - kalsilite - SiO 2 under 0.5 GPa [P(H 2 O) = P(Total)] (after Zhang
and MacKenzie 1984)

intersects thenepheline-feldspar C. Before

they determined the liquidus of this system Zhang and MacKenzie (1984) establish
that the invariant point between the

field boundary at Ne 25 Ks 51 Q 24 and 630 °

fields of nepheline, K-rich feldspar and kalsilite
to be a eutectic. Earlier, the invariant point between the

fields of K-rich feldspar,
Na-rich feldspar was also shown by Morse (1969) to be a eutectic. In that case,
there should be a temperature maximum on the nepheline-K-rich feldspar boundary
as shown in Fig. 7.5 . In order to prove the presence of such a temperature maxi-
mum, they determined the positions of two three-phase triangles on either side of
the presumed eutectic, which are shown in Fig. 7.6 . They determined these three
phase triangles by microprobe analyses of nepheline, feldspar and glass from run
products, whose bulk compositions yielded these three-phase assemblages. They
argued that the temperature maximum on the nepheline-feldspar field boundary

should lie between the trailing edges on the three phase triangles, and they estab-
lished the maximum at Ne 37 Ks 37 Q 26 . In the diagram (Fig. 7.6 ) they showed a
number of experimentally-determined conjugation lines especially in the K-rich
feldspar and kalsilite

field. These conjugation lines demonstrate the directions
toward which the liquid should move with fractional crystallization.