Pyroxenes, Pyroxenoids and Amphiboles

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

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

Pleochroism of enstatite Pale reddish Pleochroism of enstatite Pale greenish

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

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.

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

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)

Cleavage in the amphiboles

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

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

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

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

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

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

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

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”

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