1. Feldspars

2. K - Feldspars
Alkali feldspars
Alkali feldspars

3. sanidine-anorthoclase (K,Na)AlSi3O8B C D

4. These alkali feldspars are stable at high temperatures and occur in felsic volcanic rocks and some contact metamorphic rocks.  The name sanidine refers to potassic compositions (K/(K+Na) > 0.37), which are monoclinic.  More sodic compositions are termed anorthoclase which crystallize with monoclinic symmetry but invert to triclinic upon cooling, which results in polysynthetic twinning.  Both are colourless in plane light (A and C), and have maximum interference colours of first order white (B, D).  Sanidine commonly forms simple Carlsbad twins (B).  The polysynthetic twinning in anorthoclase (Albite and Pericline Laws) is superficially similar to that of microcline, but the twins are much finer in anorthoclase (D), and these two feldspars do not form in the same environment, as anorthoclase is a volcanic product and microcline occurs in slowly cooled plutonic rocks.  A and B are from a welded tuff from White River Canyon, Nevada, and C and D are from a rhyolite from an unknown location.  All views are 2.2 mm across.  A and C ppl, B and D x-nicols.

5. Plane-Polarized Light
Low relief
Clear
Cleavage often visible (A)
Usually numerous inclusions, giving it a much dustier look than quartz.
The inclusions are due to microscopic alteration (B)
Crossed Polarizers
Gray to white interference colors, slightly lower than quartz
Alteration products often have high interference colors (A)
Often has a distinctive crosshatch or "tartan" twinning pattern.(B)

6. Phase transitions in K-feldspar, KAlSi3O8
Fully Al,Si ordered K-feldspar is called microcline.
Microcline has characteristic cross-hatched twinning, seen in a polarizing microscope :
This characteristic microstructure is due to the existence of both albite and pericline twinning in the crystal which has transformed from the high temperature disordered monoclinic structure.

7. Microcline Low relief Inclusions distinctive crosshatch
or "tartan" twinning pattern
= albite and pericline twinning

8. Twin domains in microcline are spindly and somewhat wispy.
This is in contrast to the straight and generally continuous twin domains in plagioclase.

9. Carlsbad twins
of K-feldspar as
phenocrysts in
vulcanic rock

10. Myrmekite
Quartz “worms” intergrown with
microcline

11. The alkali feldspar phase diagram
Na-feldspar + K-feldspar
“Perthite”
Al,Si ordering
solvus
The disordered solid solution can only exist at high temperatures.
Below the solvus the solid solution breaks down to 2 phases - one Na-rich, the other K-rich.
This exsolution process results in a 2-phase intergrowth, called perthite

12. Cross-hatched twinning
Perthite microstructure - an intergrowth of Na-feldspar in K-feldspar
Antiperthite: K-feldspar in Na-Feldspar
Cross-hatched twinning
in K-feldspar
Na-feldspar
white

13. Refractive indices of Albite > Refractive indices of K-feldspar
Perthite:
Albite lamellae in K-feldspar
Becke lines move into lamellae
Antiperthite:
K-feldspar lamellae in Albite
Becke lines move into matrix
Refractive indices of Albite >
Refractive indices of K-feldspar

14. Plagioclase
Inclusion
and corrosions
Tiger-stripe twinning

15. Plane-Polarized Light
Low relief
Clear
Cleavage often visible (A)
Usually numerous inclusions, giving it a much dustier look than quartz. The inclusions are due to microscopic alteration (B)
Crossed Polarizers
Gray to white interference colors, slightly lower than quartz
Tiger-stripe twinning; (not all plagioclase shows twinning)
Euhedral crystals are common and frequently zoned (A).
Corroded crystals (B) are also common due to alteration or reaction with magma or other minerals
Alteration products often have high interference colors (C).

16. Twinning in plagioclase feldspars
Albite twinning is caused by the presence of pseudo-mirror symmetry parallel to (010):
Twin plane is // (010)
Components are related by reflection in (010)
Polysynthetic twinning

17. Michel-Levy
Method

18. Michel-Levy method
Determination of anorthite component in plagioclase via extinction angle
Select a grain in which the (010) plane is vertical.
Indications:
All twins have the same interference colour between crossed polarizers when placed to the N-S and in the 45° position
B. The composition planes between the twin lamellae are crisp and sharp.
C. The extinction angles measured below should differ by no more than 4 °

19. Michel-Levy Method

20. Michel-Levy Method

21. Zoning of
plagioclase crystals

22. Zoning
albite, carlsbad,
and pericline twins

23. Alkali feldspars
Oligioclase
Andesine
Labradorite
Bytowinite

24. Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
At high temperatures there is complete solid solution, involving the coupled
substitution:
Na+ + Si4+  Ca2+ + Al3+
NaAlSi3O8 albite
CaAl2Si2O8 anorthite
1100
1200
1300
1400
1500
1553
Temperature oC 20 40 80 60
melt
melt + plagioclase
plagioclase solid solution
Note: In albite the Al:Si ratio is 1:3. In anorthite it is 2:2

25. Zoning Variations in Plagioclase
Normal zoning: anorthite-rich core to albite-rich
rims
Reverse zoning: albite-rich core to anorthite-rich
Oscillatory zoning: repeated alternations between albite-rich and
anorthite-rich zones