Geology Reference
In-Depth Information
Fig. 2.12 Phase relations along the join nepheline
kalsilite determined at 2 GPa [P(H
2
O) = P
(Total)] and various temperatures. The solvus has been inferred from EPMA data for subsolidus
runs (after Gupta et al. 2010)
—
Duzakara, Bunagana and Hyamachunchu. Nepheline together with melilite and
leucite has also been reported from the Fort Portal area of Toro-Ankole by Holmes
and Harwood (1932, p. 379). It occurs in lavas of Lake Kivu (Bowen and Ellestad
1937). Nepheline-bearing rocks are also present at Villa Senni, Italy.
Nepheline occurs as a groundmass phase in Roccamon
na leucite phonolite
lavas (Melluso et al. 1996). Leucite-rich phonolitic tephrite occurs south of Lake
Barciano (Sabatini district), and are characterised by nepheline in the groundmass.
Mt. Vulture volcanics (Table
2.8
) contain nephelines with excess silica. For
example, in Prete della Schimmia, a nepheline crystal (Ne
70
Ks
25
Q
5
) with low CaO
content up to 2 wt%) have been described by Melluso et al. (1996). The SrO content
of these nephelines is low.
Melilite-bearing foidite occurring south of Caterira, comprises nephelines, which
are more Ca-rich with values ranging from 3 to 7 wt%. One particular sample
yielded a value of 8.83 wt%, corresponding to a normative anorthite content of
17 %. These nephelines are lower in K contents compared to that of the ha
yne-
nephelinites and melilitites. Melluso et al. (1996) thought that such high CaO
contents have never been reported earlier. They further observed that nephelines
from Toppo S. Paolo phonolite have high amount of silica (Ne
75
Ks
13
Qz
12
) and plot
near Barth
ü
s compositional trend: (K
2
Na
6
Al
8
Si
8
O
32
→
□
2
Na
6
Al
6
Si
10
O
32
(Dollase
and Thomas 1978).
Nepheline is a rare euhedral mineral in the leucitites from New South Wales,
Australia. These nephelines (
'
1.541) are characterised
by excess SiO
2
(Cundari 1973; Table
2.8
). It occurs as euhedral crystal in the
∈
= 1.530
1.537;
ˉ
= 1.534
-
-
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