Geology Reference
In-Depth Information
Ca
2
MgSi
2
O
7
akermanite
þ
KAlSi
2
O
6
leucite
þ
NaAlSi
3
O
8
albite
ð
Þ
ð
Þ
ð
Þ
ð
11
:
1
Þ
$
CaMgSi
2
O
6
diopside
þ
NaAlSiO
4
nepheline
Þ
þ
CaSiO
3
wollastoniteo
Þ
þ
KAlSi
3
O
8
orthoclase
ð
ð
ð
Þ
ð
Þ
The experimental results of Dwivedi et al. (2007) show that sodic-plagioclase
does not coexist with leucite. When leucite
ss
is in equilibrium with feldspar, it is
alkali feldspar with the following compositions: An
4
Ab
49.5
An
46.5,
An
3
Ab
53.5
Or
43.5,
and An
3.5
Ab
51.5
Or
45.0
.
Nepheline
ss
appearing in this system contains kalsilite and some excess silica as
albite molecule. Electron microprobe analyses show that nepheline
ss
coexisting with
leucite has the following compositions: Ne
92
Ks
7
Q
1,
Ne
90
Ks
8
Q
2,
and Ne
91
Ks
7.5
Q
1.5.
The wollastonite
ss
appearing in the subsolidus region of the join leucite-akermanite-
albite contains 1.10
1.52 wt% MgO. Melilite is present in large quantities in the
akermanite-rich side of the join, and has the following compositions: Ak
80
Geh
13
Sm
7,
and Ak
81
Geh
9
Sm
10,
where the amount of melilite is less than that of wollastons
ss
and
nepheline
ss
, which appears as an additional phase. Presence of considerable amounts
of soda-melilite and gehlenite componests in melilite is well known in leucite-
bearing ma
-
c rocks from ltaly (Meluso et al. 1996).
Le Bas (1977) described wollastonite-bearing ijolite containing signi
c and ultrama
cant
amounts of wolastonite, pyroxene, nepheline, and small amounts of feldspar with
other accessory minerals from Homa Bay. Wollastonite-bearing urtite has also been
described from the same locality. In the alkaline rocks associated with alvikites
from Omutageti-Nyamgurka complex of Kisingiri, Africa. Le Bas (1977) also
described the occurrence of mafelsic lavas containing wollastonite
ss
, nepheline
ss
,
clinopyroxene and alkali feldspar. In Usaki ijolite complex, LeBas described ijolite
and urtite containing wollastonite, nephelin, and clino- pyroxene with trace of alkali
feldspar. Le Bas (1977) observed that most of the Nyamaji phonolite dykes and
many of the lavas are characterized by the presence of alkali feldspar, alkali
pyroxene, wollastonite
ss
, and nepheline
ss
. Wollastonite-bearing nepheline syenite
and ijolite (nepheline + clinopyroxene) have also been dsscribed from north Ruri by
Le Bas (1977).
The occurrence of modal wollastonite in kamafugitic rocks has been described
by Sahama (1974) from Uganda, Africa. In addition, the association of wollastonite
along with diopside, melilite and leucite has been described by Stoppa (1988) from
Colle Fabbri, Spoleto, Italy.
11.4.2 The Join Leucite-Akermanite-Albite
50
Anorthite
50
Under Atmospheric Pressure
Twenty glasses were made to study this join at atmospheric pressure (Fig.
11.10
).
Phase relations in the join leucite-akermanite-albite and that in the join leucite-
akermanite-albite
50
anorthie
50
(Fig.
11.10
) are compared, it can be seen that when
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