ERTH 2002

1
ERTH 2002

• Mineral of the Week (MoW)
• olivine
• 2. pyroxene
• 3. feldspar (some review)
• 4. garnet
• 5. amphibole
• 6. mica
• 7. quartz
• 8. clays
• 9. carbonates (some review)
• 10. Fe-Ti oxides
• 11. sulphides (some review)

properties chemical composition crystal
structure phase diagrams occurrence use
2
Perthite
MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
photos mindat.org
3
MoW 3 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)
4
MoW 3 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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MoW 3 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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MoW 3 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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MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
a framework silicate with the general formula
AZ4O8
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)
8
MoW 3 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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MoW 3 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
10
MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
plagioclase in thin section photos from
OM-series, ERTH 2001
PPL
PPL
OM-17 (4 mm) zoned plag in a volcanic rock
OM-12 (4mm) zoned plag in a plutonic rock
XN
XN
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MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
plagioclase normal zoning from Ca-rich cores
to Na-rich rims 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
12
MoW 3 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
13
MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
alkali feldspar exsolution lamellae
(perthite) form during unmixing of
homogeneous K,Na-feldspar during slow
cooling (subsolidus solvus)
anorthoclase
distinguishing features 1 (Kfd vs plag or
qtz) simple (Carlsbad) or cross- hatched
(tartan) twins exsolution common
zoning relatively rare biaxial ve
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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MoW 3 Feldspar (Nesse, Ch. 12, p. 208-225)
occurrence most abundant mineral in Earths
crust (generally not stable in the
mantle) plag essential (dominant) mineral in
mafic and intermediate igneous rocks (basalts,
andesites, etc.) - important constituent of most
felsic and alkaline igneous rocks - present in
metamorphic equivalents of these rocks, as well
as metasedimentary rocks (marbles,
pelites) - common detrital mineral in
sedimentary rocks - present in many hydrothermal
vein deposits alkali fd essential mineral
in felsic and alkaline igneous rocks, common
in some intermediate igneous rocks - present in
some high-grade metamorphic rocks
(granitioid and pelitic gneisses,
migmatites) - common detrital mineral in
sedimentary rocks (also authigenic) - present in
many hydrothermal vein deposits (var.
adularia) use semi-precious stone (moonstone,
labradorite) aggregate, decorative
stone filler, abrasive, whitener, ceramics