Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 13

Fig. 2.1 Phase diagram of
the system
KAlSi 2 O 6 - NaAlSi 2 O 6 under
0.1 GPa P(H 2 O) (after Fudali
1963)

1400

1300

1200

L

1100

X

1000

Leucite+L
Leucite+Nepheline+L

900

Nepheline L

a

Leucite

b

800

d

c e

Leucite+

700 Nepheline

Feldspar

Nepheline+

+

Nepheline+Feldspar+L

Feldspar

500

90 80 70 60 50 40 30 20 10

NaAlSi 26

Weight (%)

KAlSi O
2

The lavas of Alban Hills, central Italy, often contain fresh leucite crystals with
near theoretical composition having 0.27 wt% Na 2 O (Table 2.1 , Francalanci et al.
1987). They are unzoned and do not have excess silica as observed in leucites from
Vico and Vulsini (Holm et al. 1982).
The Gaussberg lavas have leucite crystals, which are generally twinned indi-
cating a slow rate of cooling through the cubic-tetragonal inversion temperature.
The compositional range is small and is very close to ideal leucite stoichiometry
(Sheraton and Cundari 1980). They did not observe any signi cant difference in

chemistry (Table 2.1 ). There is also signi cant amount of Ba (up to 0.3 wt% BaO)

in the leucites of Gaussberg lamproites.
Vico lavas (Cundari 1975) have ubiquitous leucites, which show variable crystal
development within and between

flows from micro-crystalline to phenocrystal
phase (1.5 cm across). Leucite often shows signs of resorption, and in rare cases is
altered to analcite. Their K 2 O content falls within the range of 17.6

20.0 wt%. The

-

Na/K ratio varies signi cantly from 0.013 to 0.057 wt% even in leucites from the

same rock specimen. A phenocrystal core has higher Na/K ratio than the rim. This
observation is in agreement with the leucite data of Carmichael et al. (1974) from
the similar lava type of the Roman region. The Vico leucites, however, show excess
silica over the experimentally determined leucite solid solution (Fudali 1963). The
extent of NaAlSi 2 O 6 in Vico leucites varies from 1.1 to 5.0 wt% (Cundari 1975).
Excess silica in leucite has also been reported by Brown and Carmichael (1969)
from Korath Range and by Cundari (1973) from New South Wales.