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(Gupta and Edgar 1975). In their study of the join leucite-albite-anorthite under
atmospheric pressure Gupta and Edgar showed that leucite does not coexist with
pure albite, and reacts with it to produce nepheline
ss
and K
Na feldspar [(K, Na)
AlSi
3
O
8
]. Nepheline, thus appears in the albite-rich portion of the join along with
additional (K, Na) AlSi
3
O
8
.
Anorthite-rich plagioclase (up to 3 mm across) has been described from Brown
Leucitic Tuffs of Roccamon
-
na by Luhr and Giannetti (1987). Most of the pla-
gioclase crystals have compositions, plotting in the range of An
85
-
95
with a strong
mode at An
87
-
89
(Fig.
2.2
, Table
2.3
). These plagiocases have up to 1 wt% SrO.
Mt. Vulture tephrites and phonolitic tephrites contain plagioclase, which occurs
as a euhedral phase and is often enclosed within green salites together with leucite,
ha
yne and oxide minerals. In these rocks, the composition of plagioclase ranges
(Table
2.3
) continuously from An-rich (up to 81 vol%) through more sodic varieties
to anorthoclase (Melluso et al. 1996). Plagioclase is found as microlites in basa-
nites, tephrites and phonolitic tephrites. These plagioclase crystals are enriched in
Sr and Ba (up to 4 wt% SrO and 7 wt% BaO). The Toppo S. Paolo phonolites are
characterised by high concentration of Ba (7 wt% BaO and 2 wt% SrO). The
plagioclase feldspars (Or
5
Ab
55
An
40
) in minettes from Bearpaw Mountains are
found to be in equilibrium with sanidine. The associated latites from the same
locality contain plagioclase phenocrysts, which are idiomorphic and zoned
(Or
4
Ab
67
An
29
-
Or
5
Ab
68
An
27
).
The Latera Caldera of Roccamon
ΓΌ
na (Italy) is known for eruption of phonolitic
and tephri-phonolitic tuffs, which are constituted mainly of plagioclase crystals
(An
77
-
87
). These plagioclases are in general unzoned and unembayed in the wide
pumices, whereas some of the black pumice fragments of overlying units have
anorthitic plagioclases (An
72
-
93
), which exhibit concentric zoning and more pro-
nounced embayment and contain numerous minerals and glass inclusions. Plagio-
clase in Vico lavas (Cundari 1975) is occasionally glommeroporphyritic and
strongly zoned and seldom resorbed (also Perini and Conticelli 2002). It is often a
microphenocrystal phase. The plagioclase has anorthite content varying between
76.3 and 88.9 % in trachytic as well as tephritic phonolites.
In phonolitic tephrite of Ringgit Beser Complex of East Java (Edwards et al.
1991), the plagioclase feldspar coexisting in equilibrium with leucite, clinopyrox-
ene and phlogopite are anorthite-rich (An
91
).
2.4 Clinopyroxene
Diopside is the major component having variable amounts of CaFeSi
2
O
6
, CaAl
2-
SiO
6
, CaFe
3+
AlSiO
6
, NaCrSi
2
O
6
and NaFeSi
2
O
6
molecules in solid solution.
Orthopyroxene may be present in peridotite or pyroxenite xenoliths, but not in the
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