Geology Reference
In-Depth Information
Chapter 6
Chemical and Physical Constraints
for Crystallization of Feldspathoids
and Melilite in Potassium-Rich Rocks
Leucite appears as a mineral in K
rich lavas, if the SiO
2
content of the rock varies
between 44 and 56 wt% (Washington 1906). When the SiO
2
content is close to
56 wt%, it may appear but then the SiO
2
/K
2
O ratio should be below 10; in that case
the K
2
O content of
-
cient rocks
(SiO
2
< 45 wt%), leucite may appear even though the K
2
O content is slightly higher
than 1 wt%. The K
2
O/Na
2
O ratio is always greater than one in leucite-bearing
rocks, but the prohibitive tendency of high soda may be eliminated if the SiO
2
/K
2
O
ratio is low.
the rock is close to 6 wt%.
In silica-de
6.1 P
-
T Conditions Related to Leucite Stability
The P
T stability of leucite, kalsilite (as a break down product of leucite) and
melilite, which are typically found in kamafujitic rocks, is discussed in the following
sections. The f(O
2
) condition of formation of these rocks as determined by different
investigators is also described.
-
6.1.1 Stability of Leucite
The breakdown of leucite to form K-feldspar and kalsilite was studied
first by Scarf
et al. (1966) and later by Fasshauer et al. (1998, Fig.
6.1
). Breakdown of leucite
containing analcite takes place at a lower pressure than that of its pure synthetic
equivalent. The study of Scarf et al. and Fasshauer et al. (1998) explains the
absence of leucite under plutonic conditions.
Schairer (1950) studied the system nepheline
SiO
2
, and established that
sanidine melts incongruently to leucite and a silica-rich liquid under atmospheric
pressure and there is a large
kalsilite
-
-
SiO
2
(to
be discussed later, Fig.
7.1
). Later study by Tuttle and Bowen (1958, Fig.
6.2
) showed
that the
field of leucite in the system nepheline
kalsilite
-
-
field of leucite, produced as an incongruent melting phase of sanidine,
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