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


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

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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. (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"


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!


c

blue-green

olive green

b

Direction of polarizing plane

0,5 mm

amphibole,elongated with c parallel thin section

~(100) plane


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


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)!


Direction of polarizing plane

0,3 mm

amphibole


0,5 mm

amphibole

Observations:

Paragenesis:

Amphibole

Epidote

Chlorite


2 mm

amphibole

Nr. 50

Observations:

Retrograde Eclogite

Garnet

Titanite


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.


Direction of polarizing plane

0,3 mm

amphibole


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 !


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.


1 mm

amphibole

Observations:

Blue amphibole next tp

Garnet

Epidote

Titanite


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.


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


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.


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:


Direction of polarizing plane

1 mm

Orthopyroxene (Hypersthene)

Observations:

Hypersthene shows very nice pleochroism!

Note: you can work out the schematics of pleochroism


Direction of polarizing plane

1 mm

Orthopyroxene (Hypersthen)

Observations:

c is vertical


1 mm

Clinopyroxene

Observations:

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


0,5 mm

Clinopyroxene

Nr. 65

Observations:

Detail in sections c vertical.


1 mm

Clinopyroxene

Observations:

Turbid cpx, altered plagioclase and olivines with alteration rinds.


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.


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.


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.


1 mm

Aegirine,NaFe3+[Si2O6]

Nr. 26

Observations:

Sperolithic aggregates, sections //c.


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


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


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


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


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


1 mm

Titanian Augite

Observations:

Leucite

Vesicle

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


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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