Geology Reference
In-Depth Information
It is a S- and Cl-rich, leucite-melilite-bearing lava
flow containing an unusually
large amount of sodalite group of minerals (>23 vol%). Mineralogical and chemical
study of phenocrysts has led to the identi
ynes, blue lazurites
and of Cl-rich white or black noseans. X-ray diffraction (XRD) study con
cation of black ha
ü
rms the
occurrence of nosean having a low symmetry (P23). Raman spectra and XRD data
show that S is fully oxidised to SO
4
in black ha
ynes and in white noseans, while it
is partly reduced to form S
3
−
groups in blue lazurites, which also contain H
2
O
molecules. Among euhedral phenocrysts, large lazurites are only faintly zoned. All
other phases show variable core-rim chemical zoning and many phenocrysts are
partially resorbed and/or colour-zoned. Black ha
ü
ynes have highly variable S/Cl
and slightly lower SiO
2
/Al
2
O
3
ratios, larger Fe
(TOT)
contents and more compatible
trace elements than lazurites. Thin opaque nosean-sodalite rims surrounding all
crystals are interpreted as a result of rapid crystallization driven by exsolution of a
S-scavenging
ü
fluid phase.
2.13 Apatite
High REE content of potassic rocks is mainly due to the presence of apatite, which
is a very common accessory mineral. Apatites from Leucite Hills, Wyomintg
(U.S.A., Kuehner et al. 1981) contain 0.12
0.27 % La
2
O
3
, 0.41
0.65 % Ce
2
O
3
,
-
-
0.07
0.41 % Nd
2
O
3
(all in wt%) (Table
2.12
). Over 3 wt%
of Ca in some of the apatites is replaced by the Ce-group of rare earth elements.
There is often replacement of Mn, Sr and rare earth elements for Ca (Table
2.12
).
Apatites from the lamproitic rocks of West Kimberly, Australia are also charac-
terised by high REE contents.
In minettes from Highwood Mountains micro-phenocrystal apatites occurs
commonly (O
0.9 % Pr
2
O
3
and 0.24
-
-
'
Brien et al. 1991). It ranges in composition from chloro-hydroxy-
apatite to
fluor-apatite. Rare earth element content of this phase is typically high
(0.4 and 0.6 wt% La
2
O
3
and Ce
2
O
3
, respectively). Variable F, Cl and (OH) contents
in apatites imply a complex degassing history of Highwood Mountains magmatic
system.
In the groundmass glass of Brown Leucitic Tuff of Roccamon
na, Italy (Lhur
and Giannetti 1987), apatite (up to 0.55 mm across) is a common mineral. The F, Cl
and H
2
O contents of these apatites are 2.41, 0.16 and 0.58 wt%, respectively. It is
enriched in SrO (Table
2.12
).
2.14 Spinel
Jaques and Folley (1985) observed that aluminous spinels (pleonaste-hercynite)
occur as inclusions (mostly <20
m) in leucite phenocrysts of the
Fitzory area, Western Australia. These inclusions could be readily recognised in
µ
m but rarely >40
µ
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