1. Pyroxenes, Pyroxenoids
and
Amphiboles

2. Cleavage in the pyroxenes
100
All pyroxenes have symmetrical
extinctions in cross sections
Note: The cleavage is {210} in
orthopyroxenes.

3. Parallel extinction
of enstatite,
(Mg,Fe)2[Si2O6]
parallel to [100]

4. Pleochroism of enstatite
Pale reddish
Pleochroism
of enstatite
Pale greenish

5. c
Enstatite, Mg2[Si2O6]
parallel extinction
to [100]

7. Note that most of the clinopyroxenes are length slow, whereas the sodic clinopyroxenes aegirine and aegirine-augite are length fast.
Also, the true extinction angle, i.e. the maximum angle, is measured on a grain with the highest interference colour where the cleavage trace is also the sharpest.

8. Extinction angles of augite
circa 45-55°
Aegirine-Augite:
weak pleochroic
from green to yellow

9. Compositions of the amphiboles
Clinoamphibole: space group C2/m
The Ca, Fe, Mg amphiboles
Orthoamphibole: space group Pnma
General composition: A0-1B2C5[Si8O22](OH,F)2
(no A sites occupied in these compositions)

10. Cleavage in the amphiboles

11. Amphiboles
Plane polarized light:
Colour intensity correlates with Fe-contents
Crossed polarized light:
Orthoamphiboles: have upper first order
Clinoamphiboles: lower second order
Interference colours correlates with Fe-contents

12. Parallel extinction
of orthoamphiboles
parallel to [010]
Inclined extinction
of clinoamphiboles
parallel to [010]

14. Antophyllite/Gedrite, (Mg, Fe2+)5Al2[Si6(Si,Al)2O22](OH)2
Orthoamphibole:
Antophyllite/Gedrite,
(Mg, Fe2+)5Al2[Si6(Si,Al)2O22](OH)2
Colors that range from colorless
to gray to green to brown
Lower birefringence than the
monoclinic amphiboles ( upper 1st order)
Parallel extinction

15. birefringence in the lower 2nd order
Clinoamphibole
with low pleochroism:
Actinolite
Ca2(Mg,Fe)5[Si6(Si,Al)2O22](OH)2
birefringence in the lower 2nd order
Pleochroism: pale green

16. Cummingtonite (Fe,Mg )7[(Si,Al)8 O22](OH)2
Clinoamphibole with low pleochroism:
Cummingtonite (Fe,Mg )7[(Si,Al)8 O22](OH)2
Pleochroism: almost colourless
Interference colour : second order blue

18. Green Hornblende Clinoamphibole with strong pleochroism:
(Na,Ca)2(Mg,Fe2+,Al)5[(Si,Al)8O22](OH)2

19. Brown (oxy-)Hornblende,
Clinoamphibole with strong pleochroism:
Brown (oxy-)Hornblende,
(Na,Ca)2(Mg,Fe3+,Al)5 [(Si,Al)8O22](OH,O)2
Interference colours
of second order

21. Na-bearing amphiboles: anomalous blue-gray interference colors
Clinoamphibole with strong pleochroism:
Na-bearing amphiboles:
anomalous blue-gray
interference colors
e.g. Glaucophane,
Na2Mg3Al2Si8O22(OH)2
Na-bearing amphiboles:
bluish-purple pleochroism,
Hence, “blue amphiboles”

22. Glaucophane, Na2Mg3Al2Si8O22(OH)2 Interference colours of second order
Clinoamphibole with strong pleochroism:
Glaucophane,
Na2Mg3Al2Si8O22(OH)2
Interference colours
of second order
Core: colourless
Rim: Bluish-purple pleochroism
= variation in Fe-contents