K-Fedspatos

Prévia do material em texto

1
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspars
• 3 true alkali feldspars
– Microcline Triclinic
– Orthoclase Monoclinic
– Sanidine Monoclinic
• All have the same composition -
KAlSi3O8 with some substitution of Na 
& up to 5 mole % Ca
• 4th member - Anorthoclase is a mixture 
of albite and orthoclase (K,Na)AlSi3O8
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Feldspar 
Composition 
Diagram
Anorthite
Albite KAlSi3O8
No Natural 
Feldspars
Pl
ag
ioc
las
e 
Se
rie
s
Alkali Feldspar Series
Anorthoclase
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
RI
• all Alkali feldspar have the same RI.
– nD = 1.514 - 1.526
– nE = 1.518 - 1.530
– nJ = 1.521 - 1.533
• Low -ve relief compared to mounting 
cement
Tuesday, October 23, 12
1
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspars
• 3 true alkali feldspars
– Microcline Triclinic
– Orthoclase Monoclinic
– Sanidine Monoclinic
• All have the same composition -
KAlSi3O8 with some substitution of Na 
& up to 5 mole % Ca
• 4th member - Anorthoclase is a mixture 
of albite and orthoclase (K,Na)AlSi3O8
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Feldspar 
Composition 
Diagram
Anorthite
Albite KAlSi3O8
No Natural 
Feldspars
Pl
ag
ioc
las
e 
Se
rie
s
Alkali Feldspar Series
Anorthoclase
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
RI
• all Alkali feldspar have the same RI.
– nD = 1.514 - 1.526
– nE = 1.518 - 1.530
– nJ = 1.521 - 1.533
• Low -ve relief compared to mounting 
cement
Tuesday, October 23, 12
1
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspars
• 3 true alkali feldspars
– Microcline Triclinic
– Orthoclase Monoclinic
– Sanidine Monoclinic
• All have the same composition -
KAlSi3O8 with some substitution of Na 
& up to 5 mole % Ca
• 4th member - Anorthoclase is a mixture 
of albite and orthoclase (K,Na)AlSi3O8
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Feldspar 
Composition 
Diagram
Anorthite
Albite KAlSi3O8
No Natural 
Feldspars
Pl
ag
ioc
las
e 
Se
rie
s
Alkali Feldspar Series
Anorthoclase
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
RI
• all Alkali feldspar have the same RI.
– nD = 1.514 - 1.526
– nE = 1.518 - 1.530
– nJ = 1.521 - 1.533
• Low -ve relief compared to mounting 
cement
Tuesday, October 23, 12
2
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Optic Characteristics
• all alkali feldspars are colourless - non 
pleochroic
• similar indices of refraction
• low birefringence, interference colours -
up to 1st order white
• all biaxial -ve, variable 2V
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Cleavage
• All exhibit two cleavages
Р1 perfect to Πto 001
– 1 good to Œto 010
• microcline
– 001^010 = 90° 41'
• orthoclase, sanidine
– 001^010 = 90°
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• The major difference between the Alkali 
feldspars is in the arrangement or 
degree of order - disorder of the Si and 
Al in the tetrahedral sites of the lattice
• Influences the crystal system and class 
for each
• Distinctions between the various 
species are:
Tuesday, October 23, 12
2
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Optic Characteristics
• all alkali feldspars are colourless - non 
pleochroic
• similar indices of refraction
• low birefringence, interference colours -
up to 1st order white
• all biaxial -ve, variable 2V
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Cleavage
• All exhibit two cleavages
Р1 perfect to Πto 001
– 1 good to Œto 010
• microcline
– 001^010 = 90° 41'
• orthoclase, sanidine
– 001^010 = 90°
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• The major difference between the Alkali 
feldspars is in the arrangement or 
degree of order - disorder of the Si and 
Al in the tetrahedral sites of the lattice
• Influences the crystal system and class 
for each
• Distinctions between the various 
species are:
Tuesday, October 23, 12
2
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Optic Characteristics
• all alkali feldspars are colourless - non 
pleochroic
• similar indices of refraction
• low birefringence, interference colours -
up to 1st order white
• all biaxial -ve, variable 2V
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Cleavage
• All exhibit two cleavages
Р1 perfect to Πto 001
– 1 good to Œto 010
• microcline
– 001^010 = 90° 41'
• orthoclase, sanidine
– 001^010 = 90°
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• The major difference between the Alkali 
feldspars is in the arrangement or 
degree of order - disorder of the Si and 
Al in the tetrahedral sites of the lattice
• Influences the crystal system and class 
for each
• Distinctions between the various 
species are:
Tuesday, October 23, 12
3
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Microcline
• triclinic
• characterized by distinctive 
cross hatched (tartan plaid) 
twin pattern; 
• optic plane is nearly ŏ to 
010;
• Optically –ve, with 2VX = 65 
to 88°, lower values 
possible
c
b
a
X
Z
Y
OAP
OA
001
010
110
OA
~18°
~18°
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
From D. Schulze, 2004
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Orthoclase
• monoclinic
• Optically –ve, with 
2VX = 40 to ~70°;
• optic plane ŏ to 010. 
OA
X
OAP
Y c
b
Z
001
110
010
a
Tuesday, October 23, 12
Alkali Feldspar
KAlSi3O8)Microcline
Tuesday, October 23, 12
3
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Microcline
• triclinic
• characterized by distinctive 
cross hatched (tartan plaid) 
twin pattern; 
• optic plane is nearly ŏ to 
010;
• Optically –ve, with 2VX = 65 
to 88°, lower values 
possible
c
b
a
X
Z
Y
OAP
OA
001
010
110
OA
~18°
~18°
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
From D. Schulze, 2004
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Orthoclase
• monoclinic
• Optically –ve, with 
2VX = 40 to ~70°;
• optic plane ŏ to 010. 
OA
X
OAP
Y c
b
Z
001
110
010
a
Tuesday, October 23, 12
Alkali Feldspar
(K,Na)AlSi3O8)Orthoclase
Tuesday, October 23, 12
4
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Low Sanidine
• monoclinic
• Optically –ve, with 
2VX - 0 - 40°
• optic plane Œ to 010
High Sanidine
• monoclinic
• Optically –ve, with 
2VX 0 - 47°
• optic plane ŏ to 010
X
a
OA
OA
OAP
Z c
11
0
0100
01
b
Y
OA
a
X
OA
Pb
Z
b
Y
Y c
010
00
1
11
0
Low
High
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Occurrence
• Microcline
– found in plutonic rocks, granitic, granodiorite, syenites;
• not found in volcanic rocks
• Orthoclase
– in graniticplutonic rocks, generally in shallow intrusives.
•
Sanidine
– comon alkali feldspar in volcanic rocks of granitic
composition (rhyolite, trachyte)
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• Alkali Feldspars crystallized at high temperature 
generally contain a certain proportion of sodium-rich 
feldspar (albite) within them
• On cooling to lower temperatures the Na-rich feldspar 
component is no longer stable in the alkali feldspar 
host and the albite exsolves out
– Physically separates itself from alkali feldspar and forms 
exsolution lamellae which are visible in thin section
• Texture is a:
– Perthite - exsolution lamellae of Ab in K-spar host.
– Anti-perthite - exsolution lamellae of K-spar in Ab host.
• Similar process involves the exsolution of silica, 
which is in excess of some alkali feldspars to produce 
granophyric texture – an intergrowth of quartz and 
alkali feldspar.
Tuesday, October 23, 12
Sanidine-Anorthoclase
(K,Na)AlSi3O8
Tuesday, October 23, 12
4
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Low Sanidine
• monoclinic
• Optically –ve, with 
2VX - 0 - 40°
• optic plane Œ to 010
High Sanidine
• monoclinic
• Optically –ve, with 
2VX 0 - 47°
• optic plane ŏ to 010
X
a
OA
OA
OAP
Z c
11
0
0100
01
b
Y
OA
a
X
OA
Pb
Z
b
Y
Y c
010
00
1
11
0
Low
High
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Occurrence
• Microcline
– found in plutonic rocks, granitic, granodiorite, syenites;
• not found in volcanic rocks
• Orthoclase
– in granitic plutonic rocks, generally in shallow intrusives.
•
Sanidine
– comon alkali feldspar in volcanic rocks of granitic
composition (rhyolite, trachyte)
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• Alkali Feldspars crystallized at high temperature 
generally contain a certain proportion of sodium-rich 
feldspar (albite) within them
• On cooling to lower temperatures the Na-rich feldspar 
component is no longer stable in the alkali feldspar 
host and the albite exsolves out
– Physically separates itself from alkali feldspar and forms 
exsolution lamellae which are visible in thin section
• Texture is a:
– Perthite - exsolution lamellae of Ab in K-spar host.
– Anti-perthite - exsolution lamellae of K-spar in Ab host.
• Similar process involves the exsolution of silica, 
which is in excess of some alkali feldspars to produce 
granophyric texture – an intergrowth of quartz and 
alkali feldspar.
Tuesday, October 23, 12
4
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Low Sanidine
• monoclinic
• Optically –ve, with 
2VX - 0 - 40°
• optic plane Œ to 010
High Sanidine
• monoclinic
• Optically –ve, with 
2VX 0 - 47°
• optic plane ŏ to 010
X
a
OA
OA
OAP
Z c
11
0
0100
01
b
Y
OA
a
X
OA
Pb
Z
b
Y
Y c
010
00
1
11
0
Low
High
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Occurrence
• Microcline
– found in plutonic rocks, granitic, granodiorite, syenites;
• not found in volcanic rocks
• Orthoclase
– in granitic plutonic rocks, generally in shallow intrusives.
•
Sanidine
– comon alkali feldspar in volcanic rocks of granitic
composition (rhyolite, trachyte)
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• Alkali Feldspars crystallized at high temperature 
generally contain a certain proportion of sodium-rich 
feldspar (albite) within them
• On cooling to lower temperatures the Na-rich feldspar 
component is no longer stable in the alkali feldspar 
host and the albite exsolves out
– Physically separates itself from alkali feldspar and forms 
exsolution lamellae which are visible in thin section
• Texture is a:
– Perthite - exsolution lamellae of Ab in K-spar host.
– Anti-perthite - exsolution lamellae of K-spar in Ab host.
• Similar process involves the exsolution of silica, 
which is in excess of some alkali feldspars to produce 
granophyric texture – an intergrowth of quartz and 
alkali feldspar.
Tuesday, October 23, 12
Perthite
Tuesday, October 23, 12
4
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Low Sanidine
• monoclinic
• Optically –ve, with 
2VX - 0 - 40°
• optic plane Œ to 010
High Sanidine
• monoclinic
• Optically –ve, with 
2VX 0 - 47°
• optic plane ŏ to 010
X
a
OA
OA
OAP
Z c
11
0
0100
01
b
Y
OA
a
X
OA
Pb
Z
b
Y
Y c
010
00
1
11
0
Low
High
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
Occurrence
• Microcline
– found in plutonic rocks, granitic, granodiorite, syenites;
• not found in volcanic rocks
• Orthoclase
– in granitic plutonic rocks, generally in shallow intrusives.
•
Sanidine
– comon alkali feldspar in volcanic rocks of granitic
composition (rhyolite, trachyte)
©2001 G.C. FinnBrock University ERSC 2P22 – Brock University Greg Finn
Alkali Feldspar
• Alkali Feldspars crystallized at high temperature 
generally contain a certain proportion of sodium-rich 
feldspar (albite) within them
• On cooling to lower temperatures the Na-rich feldspar 
component is no longer stable in the alkali feldspar 
host and the albite exsolves out
– Physically separates itself from alkali feldspar and forms 
exsolution lamellae which are visible in thin section
• Texture is a:
– Perthite - exsolution lamellae of Ab in K-spar host.
– Anti-perthite - exsolution lamellae of K-spar in Ab host.
• Similar process involves the exsolution of silica, 
which is in excess of some alkali feldspars to produce 
granophyric texture – an intergrowth of quartz and 
alkali feldspar.
Tuesday, October 23, 12