Phase Relations in the System Leucite-Akermanite-Albite-SiO - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 342

to be anorthite- rich as observed by the Dwivedi et al. (2007). Their results on the
join leucite-akermanite-albite 50 anorthite 50 join indicate that when the anorthite
component is absent, leucite and albite react to form nepheline and alkali feldspar
with more than 45 % albite component. Over a small compositional range, melilite
can coexist with calcium-rich plagioclase as observed by them.
It has been further observed that the subsolidus assemblages in this join include
wollastonite ss , and nepheline ss , which precipitates in the akermanite-rich portion of
the join. Thus, the join leucite-akermanite-albite 50 anorthite 50 can be treated as a part
of

SiO 2 . Electron
microprobe analyses of diopside show that it contains Al 2 O 3 (up to 4.37 wt%) as
CaAl 2 SiO 6 molecules in solid solution.
In the join leucite-akermanite-albite 50 anorthite 50, the subsolidus assemblages
obtained at or below 1,150

the six-component system Na 2 O K 2 O CaO MgO Al 2 O 3 -

- - - -

C comprise diopside ss + wollastonite ss. The presence of
these subsolidus assemblages can be explained by the following reaction:

°

8Ca 2 MgSi 2 O 7
akermanite

þ 3NaAlSi 3 O 8

albite

þ 2CaAl 2 Si 2 O 8

anorthite ð Þ ð Þ

ð Þ

8CaMgSi 2 O 6
diopside

þ 2CaAl 2 SiO 6

Ca Tschermak Þ þ

3NaAlSiO 4
nepheline ð ð Þ

ð Þ

þ 8CaSiO 3

wollastonite

ð 11 : 2 Þ

ð Þ

The above reaction suggests that plagioclase coexisting with melilite is labrad-
oritic, as established by electron probe microanalyses by Dwivedi et al. (2007).
They observed that clinopyroxene contains 3.3 wt% Ca-Tschermak molecule in
solid solution. The subsolidus assemblage shows the presence of nepheline ss and
wollastonite ss in addition to melilite. They observed that melilite is a solid solution
of akermanite, sodamelilite and gehienite. Nepheline and wollastonite combines to
form sodamelilite, and is incorporated in the melilite structure.

11.4.3 The Join Leucite-Akermanite- Albite 50 Anorthite 50
at 1 GPa Under H 2 O-Saturated Condition

The liquidus phase relations of this join at 1 GPa under water-saturated conditions
at variable temperatures, were studied by Dwivedi et al. The experimental results
are summarized in Fig. 11.11 , which shows that in this join, there are only primary
phase

fields of diopside ss, plagioclase, and leucite ss, but melilite is absent. The join
is characterized by only a single four

± °

- phase point, E, occurring at 1,060 5 C and

Lc 43 Ak 22 (Ab 50 An 50 ) 35, where diopside ss plagioclase, and leucite ss are in

equilibrium.
Once again, nepheline ss appears from a reaction between leucite and an albite
component of plagioclase. Elimination of melilite at 1 GPa under H 2 O-saturated
conditions can be explained by the following reaction: