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Ca
60
60
d
e
b
c
a
f
g
40
40
Ca
60
60
20
20
Fe
Mg
k
hi j
Mg
Fe
80
60
40
20
Fig. 2.6 Clinopyroxene and olivine compositions from Brown Leucitic Tuff samples plotted on to
the lower portion of the pyroxene quadrilateral (mol %). Letters a
g and h
k indicate analyses of
-
-
clinopyroxenes and olivine respectively (after Luhr and Giannetti 1987)
the Brown Leucite Tuffs also contain diopside aggregates (Fig.
2.6
). The range of
salite composition is as follows: Ca
49
-
51
Mg
31
-
34
Fe
16
-
19
, whereas, diopside com-
position plots near Ca
47
Mg
47
Fe
6
.
High K-series rocks (leucite basanite, leucitite, tephritic leucitite, and leucite
phonolite) of Vulsini are characterised by pyroxenes, ranging in composition from
diopside to salite. The compositional plot of clinopyroxenes from leucite phono-
lites, trachyte and tephritic leucitites is shown in Fig.
2.6
a
c. These pyroxenes also
-
contain more than enough Al to balance silica de
ciency in the tetrahedral site, and
substitutional relation is of the following type: Mg
2+
+Si
4+
Al
3+
+Al
3+
.In
addition to the CaAl
2
SiO
6
molecule, the pyroxenes also contain CaFe
3+
AlSiO
6
(ferri-Tschermak
≈
s component). The pyroxenes from leucite phonolites and
trachytes (Fig.
2.7
) do not show discontinuous zoning like the ones from tephritic
leucitites.
The presence of two types of pyroxenes such as salite and diopside in some
Vulsini lavas have been ascribed to variation in f(H
2
f(O2) and f(O
2
) conditions in the
same magma chamber by Luhr and Giannetti. Barton et al. (1982) however, sug-
gested an alternate mechanism for the occurrence of diopside and salite in the same
lava
'
flow. According to them this may indicate mixing of two different types of
magmas, each characterised by two different types of clinopyroxenes. High pressure
of equilibration may be the reason for higher concentration of Na and Al
vi
in
pyroxenes (Thompson 1977) from Alban Hills leucitites. Pyroxenes from Vulsini
tephrites and leucitites however, show that diopsides or salites are poor in Na.
Clinopyroxenes from leucite-bearing rocks of Latera Caldera (Turbeville 1993) are
also either diopside or salite. Dark green salite phenocrysts with similar Ti and Fe
contents appear as phenocrysts (Table
2.4
). Estimates based on cation de
ciency
shows that diopside crystals are low in acmite and Ti-bearing components, whereas
the content of Tschermak
'
is molecule is lower in the fassaites. The groundmass
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