Geology Reference
In-Depth Information
groundmass of leucite basanites and tephrites from the Laacher See area, East Eifel,
Germany.
The potassic rocks of Ishimskii, Central Kazakistan, are characterised by
extrusive and subvolcanic facies rocks containing pseudoleucite phenocrysts in a
groundmass comprising microlites of K-feldspar, nepheline, and altered glass. In
the pseudoleucite-bearing nepheline syenites from lamproites of Central Asia
(42
E), nepheline occurs quite commonly.
This mineral (comprising up to 35 vol% of the rock) occurs in the alkali intrusive
rocks at Changit (Maimecha Kotui Province, Russia), where it occurs in association
with perovskite, aegirine-augite, apatite, K-feldspar and phlogopite.The syenites
comprise pseudoleucite (2
°
09
′
N, 74
°
59
′
6 cm in across) in a matrix of granular nepheline and
alkali feldspar. In these rocks pseudoleucite constitutes 30
-
40 vol% of the rock.
The mineralogy and chemistry of nephelines crystallizing in some alkaline rocks
of New South Wales were studied by Wilkinson and Hensel (1994). They noted
that except for a few exceptions, the nephelines are Si-rich types, whose Qz (quartz)
components exceed those de
-
ning the limits of excess SiO
2
in solid solution in the
Ne
C and 0.1 GPa P(H
2
O). They further established
that the nephelines in the basanites show only limited grain-to-grain compositional
variation. The nephelines in theralites and tinguaites from the differentiated Square
Top intrusion of New South Wales, and in the New Zealand tinguaite however,
vary significantly in Ne (nepheline), Ks (kalsilite) and Qz contents even within
individual samples. These nephelines may also be strongly zoned. They found that
the rims of zoned nephelines are enriched in Si and Fe
3+
, relative to core compo-
sitions. These zoning trends contrast with the compositional trend of successive
“
-
Ks
-
Qz
-
H
2
O system at 700
°
nepheline fractions in the Square Top sequence from thearalite to tinguaite,
where Qz content decreases. The nephelines coexist with high-temperature alkali
feldspars. Wilkinson and Hensel noted that in the Ne
bulk
”
Ks
Qz system they plot on
-
-
the Ne-rich side of the Barth compositional join, de
ned by the omission solid
solution series with end-members K
2
Na
6
Al
8
Si
8
O
32
(the Buerger ideal composition;
Ne
75
Ks
25
mol%) and
h
2
Na
6
Al
6
Si
10
O
32
(
denotes cavity cations vacancy
'
). The
h
compositions of most natural nephelines are restricted to the
ned by Ne in
the Barth join. Compositions more K-rich than the ideal composition are relatively
rare, but those lying close to the Ne side of the join are controlled by a number of
factors which include the physical conditions attending nepheline crystallization
and the chemistry of the alkaline hosts.
field de
2.9 Kalsilite
°
Hexagonal kalsilite (KAlSiO
4
) inverts to an orthorhombic variety at 850
C under
atmospheric pressure. The solid solution relationship between kalsilite and neph-
eline at, 0.5 and 2 GPa are shown respectively in Figs.
2.11
and
2.12
. The mineral
kaliophilite (another hexagonal polymorph) has a compositional range identical to
kalisilite and it may be metastable at room temperature. Kalsilite is a rare mineral in
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