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Compact Course Microscopy of rock-forming Minerals Part 4: amphiboles and pyroxenes

Compact Course Microscopy of rock-forming Minerals Part 4: amphiboles and pyroxenes. amphibole Formula : very complex group of chain silicates Symmetry : rhombic / monoclinic n : 1,6 – 1,8 n : 0,01 – 0,03 2V : highly variable max. I. F. (30 μ m) : I. – II. Order. Observations:

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Compact Course Microscopy of rock-forming Minerals Part 4: amphiboles and pyroxenes

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  1. Compact Course Microscopy of rock-forming Minerals Part 4: amphiboles and pyroxenes

  2. amphibole Formula : very complex group of chain silicates Symmetry : rhombic / monoclinic n : 1,6 – 1,8 n : 0,01 – 0,03 2V : highly variable max. I. F. (30μm) : I. – II. Order Observations: In sections with vertical c-axis typical cleavage angles {110}of 120° . Strongly coloured varieties are pleochroic Orthoamphibole green and brown amphiboles" "blue amphiboles" “colourless amphiboles"

  3. b olive green beige Direction of polarizing plane 0,5 mm a‘ amphibole,orientation with vertical c ~(001) Note: only in direction of nx and nz you will see the exact pleochroic colours, all other directions will show mixed colours!

  4. c blue-green olive green b Direction of polarizing plane 0,5 mm amphibole,elongated with c parallel thin section ~(100) plane

  5. Extinction angle Orientientation of indicatrix c blue-green beige a Direction of polarizing plane 0,5 mm amphibole,section parallel (010) Nr. 2 Position of extinction not completely black because of disperion

  6. c blue green c blue green blue green Zc b olive green b olive green olive green a beige beige beige a‘ amphibole, Schematics of pleochroism Note: determination of extinction angle ONLY possible along (010) with pleochroism blue green to beige and maximum interference colours)!

  7. Direction of polarizing plane 0,3 mm amphibole

  8. 0,5 mm amphibole Observations: Paragenesis: Amphibole Epidote Chlorite

  9. 2 mm amphibole Nr. 50 Observations: Retrograde Eclogite Garnet Titanite

  10. 0,5 mm amphibole Nr. 26 Observations: Alkali-Amphibole, extremely anomalous interference colours, strong zonation. Turbid, elongated crystals are ternary feldspars with perthitic exsolutions. Green: Aegirine.

  11. Direction of polarizing plane 0,3 mm amphibole

  12. c c ultramarine ultramarine Zc b b violett violett violett a beige beige beige a‘ amphibole, Pleochrosim of blue amphiboles ultramarine Note: determination of extinction angle ONLY possible along (010) with pleochroism ultramarine-beige !

  13. l'(-) l'(+) 0,5 mm amphibole Observations: c vertical c elongated ~(010) c elongated ~(100) Note: It is possible to determine the type of blue amphibole (glaucophane, crossite or magnesio-riebeckite) by determining the position of the indicatrix.

  14. 1 mm amphibole Observations: Blue amphibole next tp Garnet Epidote Titanite

  15. 0,5 mm Orthoamphibole Observations: Orthoamphibole with faint leochroisms and exsolutions. Orientation c vertical. Note: finding appropriate elongated cross sections (c parallel thin section plane) you can determine the extinction angle and thus the type of amphibole.

  16. 2V 90° En x Fs Pyroxene Formula : (Mg,Fe,Ca)[Si2O6] Jadeite NaAl[Si2O6], Aegirine NaFe3+[Si2O6]) Symmetry : orthorhombic/monoclinic n : 1,65 – 1,80 (1,84 for Na-pyroxenes) n : 0,01 – 0,03 (0,06) 2V : highly variable max. I. F. (30μm) : I. – III. Order Observations: In cross sections with c vertical typical shape and 90° cleavage along{110}. Orthopyroxene Diopside-Hedenbergite Augite Titanaugite Aegirine-Augite

  17. 0,5 mm Pyroxene Observations: Idiomorpic crystals in fine-grained matrix c is vertical c is horizintal Note: Many andesites have two types of pyroxene : Opx: normal extinction // c, low interference colour. Cpx: extinction at angle with c, high interference colours.

  18. 2V 90° 1 mm En x Fs Orthopyroxene (Bronzite) Observations: Orthopyroxenite. Hypidiomorphic to idiomorphic Opx (Bronzite) with exsolutions of cpx. c is vertical c is horizontal 2V of Opx depends on composition:

  19. Direction of polarizing plane 1 mm Orthopyroxene (Hypersthene) Observations: Hypersthene shows very nice pleochroism! Note: you can work out the schematics of pleochroism

  20. Direction of polarizing plane 1 mm Orthopyroxene (Hypersthen) Observations: c is vertical

  21. 1 mm Clinopyroxene Observations: Xenomorphic crystals intergrown with plagioclase, some amphibole and Fe-oxide.

  22. 0,5 mm Clinopyroxene Nr. 65 Observations: Detail in sections c vertical.

  23. 1 mm Clinopyroxene Observations: Turbid cpx, altered plagioclase and olivines with alteration rinds.

  24. 1 mm Two distinct pyroxenes Observations: cpx, opx and plagioclase. Alteration in Opx at grain boundaries and along cleavage planes. opx cpx, c is vertical opx: normal extinction //c, low interference colour. cpx: extintion not // c, High interference colour During alteration, opx will be affected more rapidly and severely compared to cpx, which often may aid identification.

  25. 1 mm Two distinct pyroxenes Observations: cpx, opx and plagioclase. Opx with higher relief and alteration. opx cpx cpx: extinction not // c, High interference colour During alteration, opx will be affected more rapidly and severely compared to cpx, which often may aid identification.

  26. 1 mm Two distinct pyroxenes Nr. 61 Observations: cpx and opx. Opx with higher relief. opx cpx: extinction not // c, High interference colour During alteration, opx will be affected more rapidly and severely compared to cpx, which often may aid identification.

  27. 1 mm Aegirine,NaFe3+[Si2O6] Nr. 26 Observations: Sperolithic aggregates, sections //c.

  28. Direction of polarizing plane 0,1 mm Aegirine,NaFe3+[Si2O6] Observations: Impressive pleochroism, perfectly developed clevage at 90° in sections vertical c. Note: You can work out the scheme of pleochroism Typical for alkaline volcanic and plutonic rocks

  29. 0,5 mm Aegirine,NaFe3+[Si2O6] Observations: Aegirine as phenocryst. Small prisms in fine-grained matrix. Aegirine c vertical Titanite Typical for alkaline volcanic and plutonic rocks

  30. 1 mm Jadeite,NaAl[Si2O6] Observations: Jadeite, high-pressure clinopyroxene, retrogression to albite following this reaction : NaAl[Si2O6] + SiO2 = NaAl[Si3O8] Garnet Jadeite c vertical retrograde replacement along grain boundaries and cleavage planes

  31. 1 mm Omphacite Observations: Omphacite replaces plagioclase at high pressures (in eclogites). Intermediate member of solid solution series between Jadeite NaAl[Si2O6]and Diopside CaMg[Si2O6]. Omphacite c is vertical Garnet

  32. 1 mm Titanian Augite • Observations: • Hour glass structure: Different segments of crystals have different compositions and thus different optical properties. When crystals grow, elemental partitioning depends on, and is different for, distinct crystallographic growth planes. Sectors are different in: • pleochroism • extinction angle • anomalous interference colours near extinction

  33. 1 mm Titanian Augite Observations: Leucite Vesicle Vesicle filled with epoxy and air bubble. Birefringence in resin due to strain.

  34. 1 mm Titanian Augite Observations: Titanian augite: large phenocrysts in finer-grained matrix. Smaller Ti-augite have highly anomalous interference colours.

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