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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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slide1

Compact Course

Microscopy of rock-forming Minerals

Part 4: amphiboles and pyroxenes

slide2

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"

slide3

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!

slide4

c

blue-green

olive green

b

Direction of polarizing plane

0,5 mm

amphibole,elongated with c parallel thin section

~(100) plane

slide5

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

slide6

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)!

slide8

0,5 mm

amphibole

Observations:

Paragenesis:

Amphibole

Epidote

Chlorite

slide9

2 mm

amphibole

Nr. 50

Observations:

Retrograde Eclogite

Garnet

Titanite

slide10

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.

slide12

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 !

slide13

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.

slide14

1 mm

amphibole

Observations:

Blue amphibole next tp

Garnet

Epidote

Titanite

slide15

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.

slide16

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

slide17

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.

slide18

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:

slide19

Direction of polarizing plane

1 mm

Orthopyroxene (Hypersthene)

Observations:

Hypersthene shows very nice pleochroism!

Note: you can work out the schematics of pleochroism

slide20

Direction of polarizing plane

1 mm

Orthopyroxene (Hypersthen)

Observations:

c is vertical

slide21

1 mm

Clinopyroxene

Observations:

Xenomorphic crystals intergrown with plagioclase, some amphibole and Fe-oxide.

slide22

0,5 mm

Clinopyroxene

Nr. 65

Observations:

Detail in sections c vertical.

slide23

1 mm

Clinopyroxene

Observations:

Turbid cpx, altered plagioclase and olivines with alteration rinds.

slide24

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.

slide25

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.

slide26

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.

slide27

1 mm

Aegirine,NaFe3+[Si2O6]

Nr. 26

Observations:

Sperolithic aggregates, sections //c.

slide28

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

slide29

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

slide30

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

slide31

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

slide32

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
slide33

1 mm

Titanian Augite

Observations:

Leucite

Vesicle

Vesicle filled with epoxy and air bubble. Birefringence in resin due to strain.

slide34

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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