Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 48

NaAlSi 2 O 6 ). Wilkinson and Hensel observed that groundmass analcites in the ba-
sanites, mugearite and tinguaites from New Zealand, have relatively constant
composition, approaching stoichiometric NaAlSi 2 O 6. They also noticed the pres-
ence of an unusually Si-rich deutic analcite (60.2 wt% SiO 2 ) in vugs of the New
Zealand tinguaites.
Wilkinson and Hensel (1994, Table 2.9 , Fig. 2.13 b) determined compositions of
nephelines and analcites from different localities of the Bearpaw Mountains. These
nephelines and analcites are from theralites [ST28 and ST20 (Fig. 2.13 a)] and
analcite tinguaites [ST40, ST16 and ST12 (Fig. 2.13 b)] from Square Top, Porto-
bello, Nombi and Spring Mount (Fig. 2.13 c) (all from New Zealand). Their com-
positions are plotted as mole proportions with reference to NaAlSiO 4

(Ne) figure, dashed line (Barth line)

denotes the Dollase-Thomas (1978) compositional trend of natural nephelines. The
solid line marks the maximum limit for solid solution of feldspar in nepheline at
1,068

KAlSiO 4 (Ks) SiO 2 (Qz) in Fig. 2.13 . In this

- -

C and 1 atmospheric pressure. This is extrapolated from the limit in the
binary Ne

°

Qz system (Greig and Barth 1938). The dashed-dot line marks the limit
of solid solution at 700

-

C, 0.1 GPa P(H 2 O) (Hamilton 1961). In the Fig. 2.13 ,
nephelines (a and b) are denoted by circles and analcites by triangles (solid symbols
refer to analyses made by Wilkinson and Hensel 1994). Analyses C

°

R (a) denote
core and rim compositions of zoned nephelines in theralites ST28 and ST20,
respectively. The point (Fig. 2.13 c, circles) refer to nephelines (denoted by solid
circles refer to analyses in Table 2.9 , Nos. 1, 2), whereas solid triangle is ground-
mass analcite (Table 2.9 , No. 5) in the Potobello analcite tinguaite. Points C and R
denote the core and rim compositions of a zoned nepheline phenocrysts. The Por-
tobello Si-rich analcite (Table 2.9 , No. 6) falls outside the plot. Solid squares denote
Nombi nephelines (Table 2.9 , Nos. 5 and 6) and open squares refer to Nombi
analcites (Table 2.9 , Nos. 7, 8) linked by the tie-lines 3

-

6. The Spring
Mount analcite (Table 2.9 , No. 9) is represented by the solid diamond symbol.

5 and 4

- -

(b)

Qz

(c)

(a)

Qz
11
12

Qz

Qz
4

50 10

NaAlSi 26

NaAlSi 26 NaAlSi 26

7

Ne 50 Ks

266 0 2
Na Al Si O

266 0 2
Na Al Si O 6

3

Portobello,
Nombi,
Spring Mount

266 0 2
Na Al Si O

Square Top
tinguaites

5

1

Na Al Si O
223 0

Na Al Si O
223 0

Square Top
theralites

Na Al Si O
223 0

2
R

2 R

1

30 30

30

1

R 7

9

C
C 1

5R C

20

20

20

8

3

6

10

4C 10

10

V V V V

V V V V V V V V

Ne

10 20 30 40

Ks Ne

10 20

Ks Ne

10 20 30 40

Ks 30 40

KNaAlSiO
2 KNaAlSiO 2

KNaAlSiO 2

6 8 8 32 6 8 8 32 688 2

Fig. 2.13 Composition of nephelines and analcites from (a) Square Top theralites (ST28 and
ST20), (b) analcite tinguaites (ST40, ST16 and ST12) and (c) Portobello, Nombi and Spring
Mount hosts plotted as mole proportions of the components NaAlSiO 4 (Ne) KalSiO 4 (Ks) SiO 2

- -

(Qz) (After Wilkinson and Hensel 1994)