Geology Reference
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from the Alto Paranaiba Igneous Province do not show any overall peculiarity, but
have chemistry similar to those of kimberlites and lamproites in terms of their TiO
2
and Al
2
O
3
contents.
The solubility of TiO
2
in phlogopites crystallizing in the simpli
ed system,
K
2
Mg
6
Al
2
Si
6
O
20
(OH)
4
-
K
2
Mg
5
TiAl
4
Si
4
O
20
(OH)
4
was
determined at variable pressures between 1 and 3 GPa and 825
K
2
Mg
4
TiAl
2
Si
6
O
20
(OH)
4
-
C, under a
condition of P(H
2
O) < P(Total) by Tronnes et al. (1985). They found a complete
substitution of the following types: Mg
VI
, 2Si
IV
1300
°
-
Ti
IV
in
phlogopite. They tried four sets of experiments with Ti-phlogopite containing 7.1,
6.3, 4.8 and 2.4 wt% TiO
2
at variable temperatures. Their study on phlogopite co-
existing with rutile and vapour indicates that Ti content of phlogopite increases with
temperature at a given pressure. Their investigation on phlogopite in equilibrium
with rutile and a vapour phase shows that it has 7.1 wt% TiO
2
at 1,275
Ti
IV
, 2Al
IV
and 2Mg
VI
≈
≈
C or higher
temperatures under 3 GPa. The same Ti-phlogopite is stable under following
conditions: 1,110
°
°
C and 2 GPa, above 1,025
°
C and 1 GPa, above 1,275
°
C and 3
GPa, above 1,110
C and 1 GPa. The Ti-
phlogopite breaks down to rutile and a vapour phase at lower temperatures.
The Orciatico lamproites (Conticelli et al. 1992) have micas, with fairly
homogeneous chemistry. They have high TiO
2
(7
°
C and 2 GPa and
finally at 1,025
°
-
8 wt%), BaO (0.2
-
1.8 wt%) and
MgO (16
40 %) and Cr
2
O
3
(0.01
-
0.06 %). The (Si + Al) and (Na + K) are insufficient to fill the tetrahedral site
(
2.2
) and interlayer (W) site, respectively. They suggested that possible substitution
as (Ti
VI, 4+
+Mg
IV, 2+
)
-
17 wt%) content and low abundance of SiO
2
(37
-
Mg
IV, 2+
+Si
IV, 4+
is more realistic (Robert 1976). They
noted positive correlation between Al and Ti and following type of substitution
involving (Ba
2+
+Al
3+
) and (K
+
+Si
4+
). The micas occurring in lamproites from
Monticatini Valdi Cecina (Italy) are strongly zoned and have wide compositional
range of TiO
2
(2
≈
7 %), MgO (15
24 %) and FeO (4
13 %). In these micas, they
-
-
-
noted following type of substitution: 2Mg
IV, 2+
Ti
IV, 4+
≈
(Forbes and Flowers
1974). Such a substitution as Mg
IV, 2+
+ 2Si
IV, 4+
Ti
VI, 4+
+ 2Al
VI, 4+
(Arima and
Edgar 1981) is also noted. The variable K/Al ratio in these micas probably re
≈
ect f
(O
2
) variation (Foley 1989). Biotite occurs rarely as phenocrysts in pumice clasts
and is intergrown within plagioclase and pyroxene. Isolated crystals are replaced by
magnetite. Altered biotite phenocrysts have Fe/(Mg + Fe) ratio varying between
0.42 and 0.21 and F content is as high as 1.14 wt%.
The leucite tephrites and tephritic leucite phonolites from Vico lavas (Cundari
1975) have micas with Fe/Mg ratio >1:2, suggesting that they are phlogopitic. Mica
in the groundmass of basic phonolitic leucite tephrite is also within the phlogopite
range [(Fe/Mg) ratio = 0.39
−
0.33]. The pleochroic scheme of mica is as follows:
α
(pale yellow) <
(dark brown to greenish brown).
Phenocrystal phlogopites in leucite-bearing lamproitic rocks of Leucite Hills are
very common minerals, but they are absent in the groundmass (Carmichael 1967). It
occurs as large polysynthetically twinned crystal with evidence of resorption in
wyomingites and orendites. Micas often are weakly pleochroic from yellow brown
to nearly colourless, but sometimes the core shows more intense pleochroism than
the rim and may include spinel. The phlogopites from Wyomingite contain up to
ʲ
=
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