Leucite- and Feldspar-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 285

9.1 Study of the System Diopside Nepheline Leucite The

- -

Join Diopside Nepheline Sanidine Under Atmospheric

- -

Pressure

The system diopside sanidine studied by of Platt and Edgar (1971;

Fig. 9.1 ) is important in establishing the paragenetic relationship between the rocks
belonging to melanephelinite series and phonolites. They established that all phases
in this system are solid solutions. If the presence of small amounts of possible Ca-
Tschermak

- nepheline -

is molecule is ignored, the join can be treated as quinary, i.e., a part of
the system NaAlSiO 4 -

'

SiO 2 . This join cuts through the pri-
mary phase volumes of forsterite ss , diopside ss , nepheline ss and leucite ss . Forsterite ss
has a reaction relation with the liquid. The

KAlSiO 4 - CaO MgO

- -

field of leucite ss is related to the
incongruent melting of sanidine ss , thus the former should react with the silica-rich
liquid at low temperatures to form alkali feldspar. In the KAlSi 3 O 8 -poor part of the
system there is a primary phase field of forsterite ss . The assemblages near the four

phase points and below the solidus, consist of the following phases:

1. nepheline ss - diopside ss - leucite ss - l (point B; Fig. 9.1 ),

2. forsterite ss - l (point A; Fig. 9.1 ),

3. nepheline ss + diopside ss + leucite ss + alkali feldspar (between 950 and 1,000

nepheline ss - diopside ss -

° C

C leucite is eliminated.
4. nepheline ss + diopside ss + alkali feldspar (below 900

central portion) and below 900 °

° C), and

5. nepheline ss + diopside ss + melilite (near the nepheline diopside join containing

-

<5 wt% sanidine).

Assemblages (1 - 5) correspond to nepheline leucitite, olivine nephelinite,

nepheline - leucite phonolite, nepheline phonolite, and melilite - nephelinite,

respectively. After elimination of leucite and appearance of K - Na feldspar, the

Fig. 9.1 Phase relation in the
system diopside nepheline

-
sanidine under atmospheric
pressure (after Platt and Edgar
1971)

-