Geology Reference
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fresh leucites, but could also be observed in lamproites, where the leucite is
replaced and pseudomorphed by K-feldspar, zeolites etc. The alumina spinels
(Table 2.13 ) occur as amoeboid-shaped to strongly rounded larger crystals. The
spinel inclusions are found to be low in Cr content (<0.2 %, commonly <0.1 %
Cr 2 O 3 ). According to Jaques and Foley (1985) these spinels have highly contrasting
compositions with high Cr content in the titanium magnesio-chromites occurring in
the groundmass (Fig. 2.15 ).
Pleonaste, the Fe-rich variety of spinel, was found in the wyomingites from
Leucite Hills (Table 2.13 ) by Kuehner et al. (1981). They also described the
presence of magnesio-chromite (Table 2.13 ) having high Fe 2 O 3 /FeO ratio and low
TiO 2 content.
Jaques and Foley (1985) have reported the occurrences of titano-magnetites
(Table 2.13 ) as euhedral or skeletal crystals in all K-rich volcanic rocks of Western
Australia, where it is found either as a late stage oxidation product of olivine, mica,
K-richterite or clinopyroxenes. Ti-rich magnesio-chromite has also been described
from the lamproitic rocks of Fitzroy basin, Western Australia (Jaques and Foley
1985).
Perovskite from minettes of Highwood Mountains (O
Brien et al. contains
12.8 % TiO 2 (Table 2.13 ), which is equivalent to 36.7 mol% of ulvospinel.
'
Fig. 2.15 Compositions of
aluminous spinel inclusions in
leucite in West Kimberley
leucite lamproites contrasted
with groundmass titanian
magnesio-chromites in terms
of Al/(Al + Cr + Fe 3+ ) versus
Mg/(Mg + Fe 2+ ) (after Jaques
and Foley 1985)
1.0
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1.0
Mg/(Mg+Fe )
2+
 
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