Transcript Lecture 20 - Silicates III

Melt-crystal equilibrium 1
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Magma at composition
X (30% Ca, 70% Na)
cools  first crystal
bytownite (73% Ca,
27% Na)
This shifts the
composition of the
remaining melt such that
it is more Na-rich (Y)
What would be the next
crystal to precipitate?
Finally, the last bit
would crystallize from Z
X
Y
Z
Melt-crystal equilibrium 2 - miscibility
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2 component mixing
and separation 
chicken soup analogy,
cools and separates
Fat and liquid can
crystallize separately if
cooled slowly
Miscibility Gap – no
single mineral is stable
in a composition range
for x temperature
monalbite
anorthoclase
1100
Temperature (ºC)

high albite
900
700
500
sanidine
intermediate albite
orthoclase
low albite
microcline
Miscibility Gap
300
10
Orthoclase
KAlSi3O8
30
50
% NaAlSi3O8
70
90
Albite
NaAlSi3O8
Ternary Diagrams
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Describe
variations of 3
components in
crystal group(s)
A20B50C30
Y
X=A50B30C20
Y=A10B10C80
Z=A30B30C40
Z
X
Combining phase and composition
diagrams for mineral groups
Mica ternary
Muscovite
KAl2(AlSi3O10)(OH)2
No micas
Miscibility Gap
Biotite series
Annite
KFe3(AlSi3O10)(OH)2
Phlogopite
KMg3(AlSi3O10)(OH)2
Feldspar Group
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Most abundant mineral in
the crust  6 of 7 most
common elements
Defined through 3 endmembers 
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Albite (Na), Anorthite (Ca),
Orthoclase (K)
Comprised of 2 series:
Albite-anorthite (Na-Ca)
 Albite-orthoclase (Na-K)
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Tectosilicates
Feldspars
Substitute Al3+
for Si4+ allows
Na+ or K+ to be
added
Albite-Orthoclase
Substitute two
Al3+ for Si4+
allows Ca2+ to be
added
Albite-Anorthite
Albite: NaAlSi3O8
Feldspar Group – Albite-Anorthite series
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Complete solid solution  Plagioclase
6 minerals
 Albite (Na)
 Oligoclase
 Andesine
 Labradorite
 Bytownite
 Anorthite (Ca)
Albite-Anorthite double duty
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End-members (Pure Na or Ca)
Minerals 90-99.99% Na or Ca
Notation:
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AnxAby  An20Ab80=Oligoclase
Feldspars
Feldspar Group – Albite-Anorthite series
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Optical techniques to
distinguish between
plagioclase feldspars:
Michel-Levy Method – uses
extinction angles of
twinned forms to determine
An-Ab content
 Combined Carlsbad-Albite
Method  uses MichelLevy technique for both
sides of a twin form
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Michel-Levy
chart for Kfeldspar
optical ID
Staining technique
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Stains that attach
to K really well
(Like Co(NO3)2 )
will higlight the Kfeldspars quickly
and easily in hand
specimen or thin
section
Tectosilicates
Feldspars
Substitute Al3+
for Si4+ allows
Na+ or K+ to be
added
Albite-Orthoclase
Substitute two
Al3+ for Si4+
allows Ca2+ to be
added
Albite-Anorthite
Albite: NaAlSi3O8
Feldspar Group – Albite-Orthoclase series
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Several minerals – Alkali Feldspars
High – T minerals
 Sanidine
 Anorthoclase
 Monalbite
 High Albite
Low Temperature
exsolution at solvus
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Chicken soup
separation
Forms 2 minerals, in igneous
rocks these are typically
intergrowths, or exsolution
lamellae – perthitic texture
monalbite
anorthoclase
1100
Temperature (ºC)

high albite
900
700
500
sanidine
intermediate albite
orthoclase
low albite
microcline
Miscibility Gap
300
10
Orthoclase
KAlSi3O8
30
50
% NaAlSi3O8
70
90
Albite
NaAlSi3O8
Alkali Feldspar Exsolution
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Melt cools past solvus
(line defining
miscibility gap)
Anorthoclase, that had
formed (through
liquidus/solidus)
separates (if cooling is
slow enough) to form
orthoclase and low
albite
In hand sample –
schiller effect  play
of colors caused by
lamellae
Liquid
1100
monalbite
anorthoclase
Temperature (ºC)

900
high albite
sanidine
intermediate albite
700
500
orthoclase
low albite
microcline
Miscibility Gap
300
10
Orthoclase
KAlSi3O8
30
50
% NaAlSi3O8
70
90
Albite
NaAlSi3O8
Alkali Feldspar lamellae
Feldspathoid Group
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Very similar to
feldspars and zeolites
Include Nepheline,
Analcime, and Leucite
Also framework
silicates, but with
another Al substitution
for Si
Only occur in
undersaturated rocks
(no free Quartz, Sipoor) because they
react with SiO2 to form
feldspars
Feldspathoids, Cont.
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Nepheline
Important
feldspathoid
mineral
Indicates
undersaturated
magma