Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Compact Course

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum

Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei

Compact Course

Microscopy of rock-forming Minerals

Part 2: Quartz and Feldspars



I: Quartz and Feldspar



QuarzFormula : SiO2

Symmetry : trigonal

n : 1,544 – 1,553

n : 0,009

2V : -

max. I. F. (30μm) : white I

Important rock-forming mineral, easy to recognise and to use to calibrate the thickness of thin sections. Quartz in standard thin sections (i.e. 30μm ) should have maximum interference colours (I.F.) of white I.

If I.F. of quartz is slightly yellow, this means that the thin section is thicker than the standard 30μm.



Observations

Maximum interference colour optical axis of mineral vertical to the optical axis of the microscope: light grey

Lowest I.F. colours, optical axis of mineral parallel to axis of microscope

Practice: compare different orientations of optical axis (based on I.F. colours) with morphology of crystals

Quartz, typical orientations

1 mm



Observations:

I.F.: black, isotropic

Conoscopic interference figure :

Uni-axial, positive

Morphology:

hexagonal

Quarz, optical axis vertical and parallel to axis of microscope

0,3 mm red I

red I



Quartz, maximum interference colour

Observations:

Symmetrical extinction and NOT parallel extinction.

All uniaxial minerals have parallel extinction with optical axis parallel to prismatic crystal faces (applies to all tetragonal, hexagonal, trigonal crystals !)

THERFORE: observed crystal faces are not prisms but must by pyramid faces !

Observations:

Symmetrical extinction and NOT parallel extinction.

All uniaxial minerals have parallel extinction with optical axis parallel to prismatic crystal faces (applies to all tetragonal, hexagonal, trigonal crystals !)

THERFORE: observed crystal faces are not prisms but must by pyramid faces !

0.5 mm

{1120}

{1122}



Quartz, Morphology

Hexagonal crystal

C vertical

Hexagonal crystal

C horizontal

Observations:

High-quartz- morphology typically observed in SiO2-rich volcanic rocks. Quarz in plutonic and metamophic rocks xenomorphic.

{1120}

{1122}Optical indicatrixOptical indicatrix



Quartz, Morphology

Hexagonal crystal

C vertical

Hexagonal crystal

C horizontal

Observations:

High-quartz- morphology typically observed in SiO2-rich volcanic rocks. Quarz in plutonic and metamophic rocks xenomorphic.

{1120}

{1122}Optical indicatrixOptical indicatrix



1 mm

Quartz, irregular extinction due to deformation



0.5 mm

Quartz, typical grain boundaries I Observations:

Irregular extinction, complex grain boundaries

Dynamic RecrystallisationDynamic Recrystallisation: formation of small-grained domains with slightly different crystallographic orientation and crystal growth and recrystallisation in zones of high concentration of crystal defects due to deformation.



Quartz, typical grain boundaries II

Observations:

Grain boundaries straight at 120° angles; no irregular extinction:

Indicative of successful reduction of surface energies through complete recrystallization.

Static Recrystallisation:Static Recrystallisation:

Recrystallisation at high T Recrystallisation at high T without deformationwithout deformation

0.1 mm



Quartz in sediments

Observations:

Authigenic growth in pore sapce of a quartz arenite. Shape of old rounded detrital grains can be detected and distinuished from later overgrowth. Old and new growth quartz have identical optical orientation (indentical birefringence).

0.5 mm

New grown quartz can have idiomorphic crystal faces wherever pore space allows.



Chalcedony Nr. 29

0,3 mm

Observations:

"Brewster Crosses" indicating spherulithic fiber growth



Chalcedony, Disctinction between Quartzine and Chalcedony ss

QuartzineLong axis of indicatrix // fibers

(Elongation): l' (+)

Chalcedony ssLong axis of indicatrix at 90° to

fibers

(Elongation): l' (-)

0,3 mm



K-FeldsparFormula : KAl[Si3O8]

Symmetry : monoclinic / triclinic

n : 1,518 – 1,532

n : 0,005 -0,007

2V : 0°-80°

max. I. F. (30μm) : light grey I

Observations:

K-spar lowest refractive index among feldspar group. Important for the detection of perthite exsolutions.

Optical properties (2V, orientation of indicatix axis w/respect to crystallographic axes) depend on the degree of Si-Al ordering in structure.



Sanidine

1 mm

Observations:

Karlsbade twins, intermediate 2V, axial plane of indicatrix // 010 low degree of Si-Al ordering fast cooling

Fine-grained matrix between phenocrysts has fluidal texture.



Nesse (2001) nach Steward & Ribbe (1983). Feldspar mineralogy, Rev. Mineral., MSA

Optical properties of K-spars depend on Si-Al ordering in structure

Nesse (2001) after Su et al. (1984). Am. Mineral. 69, 440-448

High-Sanidine: Dispersion at an angle

Microcline: horizontal Dispersion

(010

)(0

10)

(010

)

(010

)

Intermediate Si-Ar ordering: pseudo uniaxial

However, such consocopic images However, such consocopic images are only seare only seeen in thick sanidine n in thick sanidine

crystals specifiallcalycrystals specifiallcaly



Sanidine

1 mm

Observations:

Interstitial filling with low refractive index betwen idomorphic sanidines with Karlsbad twinning:

2V very small (pseudo-uniaxial)

intermediate degree of Si-Al ordering

relatively slow cooling



Microcline, Domains of twins form typical crossed pattern Nr. 55

0,3 mm

Observations:

Diffuse changes in extiction angle, no sharp boundareis between twins, unlike as are observed for plagioclase.

Optical axis image diffuse because of overlapping domains



Microcline, Myrmekite at Contact to PlagioclaseObservation:

Myrmekite are worm-shaped intergrowth patterns from replacement of K-feldspar by albite-rich plagioclase and quartz.

Typical for granitic rocks.

0,3 mm



0,1 mm

Microcline, perthitic Exsolutions

Observation:

Exsolutions always indicate difussive transport and thus slow cooling, e.g. in granitic rocks.



Alkalifeldspar, Mesoperthite

0,3 mm

Observation:

Plagioclase has higher refractive index than K-spar and thus plagioclase has a higher „relief“ (watch „Becke-Line“



Alkalifeldspar, Fibrous Perthite

0,2 mm



Alkalifeldspar, Fibrous Perthite, Detai

0,05 mm



Antiperthite

0,3 mm

Observation:K-spar exsolutions in Ab-rich plagioclase



PlagioclaseFormula : Na[AlSi3O8] – Ca[Al2Si2O8]

Symmetry : triclinic

n : 1,529 – 1,588

n : 0,007 – 0,013

2Vx : 50° – 105°

max. I. F. (30μm) : white to pale yellow I

Observations:

Important rock-forming mineral, chemical composition highly variable, reflecting conditions of growth.

Orientation of indicatrix correlates with An - Ab variations, thus optical determination of plagioclase composition is relatively easy ! Measure angle between extinction and (010), the latter is the plane of albite twinning

Albite, volcanicAlbite, volcanic

Albite, plutonicAlbite, plutonicAnorthiteAnorthite



Plagioclase, Albite twinning

(010)(010)

(010

)(0

10)

(010)(010)

Twin plane : (010)

Twin axis: (010)



(010

)(0

10)

[001][001]

Twin plane: (010)

Twin axis: [010]

Plagioclase, Karlsbad-Twin



Plagioclase, How to recognise Albite twins :

Conditions to be met:

1. Both twins have identical interference colours when parallel N-S or E-W (i.e. parrallel to ploarizing planes).

2. At 45°- Position of lamella, twin plane no longer apparent (identical refractive index).

3. Both sets of twins have symmetrical extinction angle .



Plagioclase, How to determine An - Ab composition

I. after Rittmann (Maximum extinction angle against (010)), Part 1

What to do ?

1. Find a crystal with orientation (010) ~ to plane of thin section by finding sharp traces of twin planes.

2. Verify albite twinning (see last slide).

3. Determine angle of extinction statistically: measure many, take average of maximum values found.

4. NOTE: Only the maximum angle Only the maximum angle represents the crystal with the correct represents the crystal with the correct orientation and will yield meaningful orientation and will yield meaningful results.results.Both sets of twins have

identical interference colours when parallel to plane of ploarization (N-S, E-W).

At 45°- positions twins can no longer be observed, twin planes blurred.

Your are not done yet, keep goingYour are not done yet, keep going



Perikline twinPerikline twin

cleavagecleavage

Angle Angle < 90°< 90°

Angle Angle >90°>90°

(010)(010)

(001)(001)



Michel-Levy Rule:

nx' falls into the smaller (<90°) angle between (010) and (001): (red with compensator)

nx' falls into the larger (>90°) angle between (010) and (001): (blue with compensator)

1. (010) is the trace of the albite twin intergrowth, which you need to find.

2. (001) is paralel to the trace of cleavage or the trace of intergrowths or pericline twins.

3. Tue direction of maximum and minimum refraction is determined by the position with complete extinction.

4. At 45° nx' and nz' can be determined with

the compensator (red I).

nnxx''

nnzz''

This Plagioclase hapens to be This Plagioclase hapens to be positive in the sense of the positive in the sense of the Michel-Levy rule. Michel-Levy rule.

Note, this has nothing to do with Note, this has nothing to do with the optical sign of the mineral!the optical sign of the mineral!

Keep going !!Keep going !!





aus Tröger, 1971, p. 129

• Angel measured here : 40°

• Tue crystal is positive in the sense of the Michel-Levy rule.

• Tue rock is plutonic, take solid line (dashed line for volcanic plagioclase)

The composition of The composition of this plagioclase is this plagioclase is AnAn6767!!!!




II. Method after Moorhouse (Extintion angle at Albit-Karlsbad twins)

Advantage: you need only Advantage: you need only oone crystal, no ne crystal, no statitics involved.statitics involved.

Disadvantage: approrpiate crystals with these Disadvantage: approrpiate crystals with these comnbined albite-karlcomnbined albite-karlssbad twins rabad twins rarre and e and generally only in plutonic rocks.generally only in plutonic rocks.

What to do:

1. Find an appropriate crystal with combined albite-karlsbad twins : at 45°-position, kalrsbad twins have maximum difference in interference colours with distinct shades of grey.

2. Determine the angle of extiction separately for both twins, giving two different values (unlike in the Rittman method).

1122




II. Method after Moorhouse (Extintion angle at Albit-Karlsbad twins)

aus Moorehouse, 1959, p. 59

Value obtained here:

• 40° large angle (twin )

• 18° small ange (twin 2)

Composition of the Composition of the plagioclase is Anplagioclase is An70 70 !!!!



...... However, volcanic plagioclase crystals grow during cooling, convection and replenishment of magma reservoirs and are often complexly zoned... In these cases you will

need an electron microprobe to understand and interpret plagioclase compositions. In fact, their „growth-rings“ record what happened in the magma chamber prior to eruption !!

Composition of the Composition of the plagioclase is Anplagioclase is An70 70 !!!!

...... Crystals grow during cooling from a magma and thus their „growth-rings“ record what happened in the

magma chamber prior to eruption !!

Documents

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Compact Course