ERTH 2001: LAB TS5A

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ERTH 2001 LAB TS-5A
Identification of feldspars Determination of
plagioclase composition Estimating modal
abundance () minerals
LAB TS-5B Identification of unknown
minerals (OM-16-20) LAB TS-5C Common mineral
associations
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Perthite
LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
photos mindat.org
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AT4O8 where A Ca, Na, K T Al, Si 2 solid
solution series
plagioclase CaAl2S12O8 ? ? NaAlSi3O8
anorthite (An) albite (Ab)
An0-10 albite An50-70 labradorite An10-30
oligoclase An70-90 bytownite An30-50
andesine An90-100 anorthite
alkali feldspar NaAlSi3O8 ? ? KAlSi3O8
albite (Ab) orthoclase (Or)
Or100-35 orthoclase Or35-10
anorthoclase Or10-0 albite 3 K-feldspar
polymorphs sanidine, orthoclase, microcline
(high T ?
low T)
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AT4O8 where A Ca, Na, K T Al, Si
Or100
Or35-100
feldspar nomenclature and solid-solution
limits very limited solid soution between K- and
Ca- end members
Or10-35
An100
Ab100
An10-30 An30-50 An50-70 An70-90
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AT4O8 where A Ca, Na, K T Al, Si
crystal structure of feldspar
SiO44- groups T site (Al, Si)
crankshaft arrangement
crankshafts linked by A-site cations K, Na, Ca
(CN 9)
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AT4O8
plagioclase CaAl2S12O8 (An)
NaAlSi3O8 (Ab)
triclinic biaxial /- extinction angle
changes with composition
andesine (An30-50)
albite (An0-10)
interference colours (slight increase with
Ca-content)
plag
labradorite (An50-70)
anorthite (An90-100)
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
plagioclase in thin section photos from
OM-series, ERTH 2001
PPL
PPL
OM-15 (4 mm) zoned plag in a volcanic rock
OM-12 (4mm) zoned plag in a plutonic rock
XN
XN
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
plagioclase normal zoning from Ca-rich cores
to Na-rich rim reflects crystallisation from a
fractionating magma
distinguishing features (plag vs Kfd or qtz)
polysynthetic (barcode) twinning zoning
common exsolution rare biaxial ve (An0-20)
or ve (An20-100), 2V large
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AT4O8
alkali feldpsar KAl3S13O8 (Or)
NaAlSi3O8 (Ab)
biaxial ve monoclinic or triclinic
orthoclase (high-moderate T, monoclinic)
sanidine (high T, monoclinic)
alk fd
interference colours
microcline (moderate-low T, triclinic)
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
ordering in feldspar structure
both Si and Al occupy tetrahedral (T) sites (31
in K-Na- feldspar, 21 in Ca-plag) at high T,
random distribution of Si, Al disordered
monoclinic symmetry (sanidine, orthoclase) at low
T, non-random distribution of Si, Al ordered
triclinic symmetry (microcline)
sanidine orthoclase microcline
3 K-feldspar polymorphs distinguished by degree
of ordering of Si, Al in crystal lattice
increasing order
25 Al, 75 Si 50 Al, 50 Si 100 Al 100 Si
increasing order, decreasing T
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
a) b)
twinning in feldspar
a) albite twins reflection on (010)
(polysynthetic or barcode twinning in plag) b)
pericline twins rotation on 010 (also very
common in plagioclase) c) Carlsbad twins
rotation on 001 (typical simple twins in
K-feldspar) d) Manebach twins reflection on
(001) e) Braveno twins reflection on
(021) cross-hatched (tartan) twinning in
microcline albite pericline produced during
inversion from orthoclase
c)
d)
e)
Nesse, Fig. 12.11
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
K-feldspar in thin section photos from
OM-series, ERTH 2001
PPL
OM-12 (8 mm) orthoclase in a plutonic rock note
simple twin (Carlsbad), exsolution lamellae of
Na-fd in K-fd host
XN
XN
DB-16 (2.5 mm) microcline in a plutonic rock note
very coarse exsolution lamellae
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
alkali feldspar exsolution lamellae
(perthite) form during unmixing of
homogeneous K,Na-feldspar during slow cooling
distinguishing features 1 (Kfd vs plag or qtz)
simple (Carlsbad) or cross-hatched (tartan)
twins exsolution common zoning relatively
rare biaxial ve (generally large 2V)
distinguishing features 2 (sanidine vs
orthoclase vs microcline) sanidine small
2V, simple twins orthoclase large 2V,
simple twins microcline large 2V, tartan
twins
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
plagioclase normal zoning from Ca-rich cores
to Na-rich rim reflects crystallisation from a
fractionating magma
distinguishing features (plag vs Kfd or qtz)
polysynthetic (barcode) twinning zoning
common exsolution rare biaxial ve (An0-20)
or ve (An20-100), 2V large
alkali feldspar exsolution lamellae
(perthite) form during unmixing of
homogeneous K,Na-feldspar during slow cooling
distinguishing features 1 (Kfd vs plag or qtz)
simple (Carlsbad) or cross-hatched (tartan)
twins exsolution common zoning relatively
rare biaxial ve (generally large 2V)
distinguishing features 2 (sanidine vs
orthoclase vs microcline) sanidine small
2V, simple twins orthoclase large 2V,
simple twins microcline large 2V, tartan
twins
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Determination of plagioclase composition using
the Michel-Levy method (Lab Manual p. 57 Nesse,
Fig. 12.15, 12.16)
step 1
record maximum average value
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Determination of plagioclase composition using
the Michel-Levy method (Lab Manual p. 57 Nesse,
Fig. 12.15, 12.16)
plot maximum average value read An
30
An55
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Determination of plagioclase composition using
the Michel-Levy method (Lab Manual p. 57 Nesse,
Fig. 12.15, 12.16)
plot maximum average value read An
30
?
10
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Determination of plagioclase composition using
the Michel-Levy method (Lab Manual p. 57 Nesse,
Fig. 12.15, 12.16)
plot maximum average value read An for
angles lt20 determine optic sign and relief
30
?
10
?
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Estimating modal abundance (volume ) (Lab
Manual p. 55-56)
volume of different minerals in rock referred
to as modal , modal abundance, or simply
"mode" essential for rock classification (ERTH
2002) a) visual estimate (this term) b) point
counting (next term)
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Estimating modal abundance (volume ) Lab Manual
p.70 chart for visual estimates of modal
50
80
1
20
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Estimating modal abundance (volume ) point
counting with a superimposed grid
sample OM-12, PPL, with grid identify and count
all minerals at grid points work out
sample OM-12, XPL, with grid may need to switch
back forth between PPL and XPL to identify
minerals
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Rock classification using modal abundance QAP
diagram (granitoid rocks)
Lab Manual Fig.5.1 p.56
e.g. rock with 20 quartz 50 plag 15 Kfd 15
biotite
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Rock classification using modal abundance QAP
diagram (granitoid rocks)
Lab Manual Fig.5.1 p.56
e.g. rock with 20 quartz 50 plag 15 Kfd 15
biotite
Q 23.5 A 17.6 P 58.8 P/(AP) 76.9
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Rock classification using modal abundance QAP
diagram (granitoid rocks)
Lab Manual Fig.5.1 p.56
e.g. rock with 20 quartz 50 plag 15 Kfd 15
biotite
Q 23.5
Q 23.5 A 17.6 P 58.8 P/(AP) 76.9
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Rock classification using modal abundance QAP
diagram (granitoid rocks)
Lab Manual Fig.5.1 p.56
P/(AP) this value is plotted radially from
base to Q apex
e.g. rock with 20 quartz 50 plag 15 Kfd 15
biotite
Q 23.5
Q 23.5 A 17.6 P 58.8 P/(AP) 76.9
P/(AP) 76.9
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LAB TS-5A Feldspar (Nesse, Ch. 12, p. 208-225)
Rock classification using modal abundance QAP
diagram (granitoid rocks)
Lab Manual Fig.5.1 p.56
rock is a granodiorite
e.g. rock with 20 quartz 50 plag 15 Kfd 15
biotite
Q 23.5
Q 23.5 A 17.6 P 58.8 P/(AP) 76.9
P/(AP) 76.9
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