Geology Reference
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2.46 % F. The high F (2.16 wt%) content has also been found in high K-rich
Spanish lamproites. The concentration of Ti is particularly high in phlogopites from
leucitic rocks of Australia and Spain in contrast to those from Leucite Hills (Car-
michael 1967, Table
2.5
).
The Gaussberg leucitites have micas, which are late crystallizing phases
(Sheraton and Cundari 1980). They are also titaniferous but high in BaO (1.0 wt%)
and similar to those in Smoky Butte (Velde 1975) and West Kimberley (Carmichael
1967), but poorer in Al. There is more Al atom for 22 oxygens, and much of the Ti
is assumed to be in the tetrahedral site.
2.6 Amphibole
Cross (1897) reported for the
first time unusual pleochroism in amphiboles (lemon
yellow to pink or reddish pink) from the lamproitic rocks of Leucite Hills. Similar
amphiboles have also been described from the potassic rocks (jumillite) of south-
eastern Spain (Wagner and Velde 1986) and West Kimberley, Australia (Jaques
et al. 1984). They were originally described as magnophorite, but are essentially a
variety of K-richterites [(Na, K) Na,Ca (Mg,Fe
2+
)
5
Si
8
O
22
(OH)
2
]. K-richterites
belong to the calc-alkaline amphibole group, and have almost one Fe
2+
atom in the
C-site. Richterites found in Shiprock and Sisco lamproites however, are richer in
iron varying between Mg
2
Fe
3
and Mg
3
Fe
2
(Wagner and Velde 1986). The A site
(Na + K) is over
lled [(Si + Al) < 8)]. The
remaining tetrahedral site presumably contains Fe
3+
, but in Calasparra lamproites,
even after assigning Si, Al and Fe
3+
, the tetrahedral site remains incomplete.
The Ti content of richterites in lamproitic rocks are usually high in kaersutites
(Yagi et al. 1975), and the average is around 2 wt%, but up to 9 wt% is noted in
Moon Canyon lamproites (Wagner and Valde 1986). In amphiboles with high Ti
concentration (Cancarix, Calasparra and Moon Canyon), the presence of titanium
creates a vacancy in the structure as can be seen in the diagram representing the
variation of Ti versus the sum of cations (Fig.
2.10
a and b). The P
lled and the tetrahedral sites are under
Ti stability of
-
kaersutite has been studied by Merill and Wyllie (1975).
Ti-de
cient amphiboles display a negative correlation between the Ti content and
number of (Si + Al) ions in the tetrahedral site. There is no relationship however,
between Ti contents and sum of cations. There are some amphiboles, which are
zoned. For example, the Cancarix (Spain) and Australian lamproites show darker
colour towards the rim of the grains, which corresponds to an increase in Ti and Fe
2+
and decrease in Mg. The observation may be related to the following types of sub-
stitution: (
+Ti+Fe
2+
)
≈
3 Mg. Slightly Ti-rich K-richterite coexists with
phlogopite, clino-pyroxene and leucite in the leucitites of New South Wales (Cundari
1973). The formula of these amphiboles is as follows: (Na,K)
2.0
Ca
0.8
-
0.9
(Mg, Fe
2+
,
Mn, Fe
3+
Ti)
5
(Si, Al, Ti)
8
O
22
(OH)
2
. In comparison to the richterites occurring inWest
Kimberley rocks, those present in the leucitites from New South Wales are more
sodium rich.
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