Compact Course Microscopy of rock-forming Minerals Part 2: Quartz and Feldspars

1. Compact Course Microscopy of rock-forming Minerals Part 2: Quartz and Feldspars

2. I: Quartz and Feldspar

3. Quarz Formula : 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.

4. 1 mm Quartz, typical orientations 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

5. red I 0,3 mm Quarz, optical axis vertical and parallel to axis of microscope Observations: I.F.: black, isotropic Conoscopic interference figure : Uni-axial, positive Morphology: hexagonal

6. 0.5 mm 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 !

7. Optical indicatrix Quartz, Morphology Observations: High-quartz- morphology typically observed in SiO2-rich volcanic rocks. Quarz in plutonic and metamophic rocks xenomorphic. Hexagonal crystal C vertical Hexagonal crystal C horizontal

8. Optical indicatrix Quartz, Morphology Observations: High-quartz- morphology typically observed in SiO2-rich volcanic rocks. Quarz in plutonic and metamophic rocks xenomorphic. Hexagonal crystal C vertical Hexagonal crystal C horizontal

9. 1 mm Quartz, irregular extinction due to deformation

10. 0.5 mm Quartz, typical grain boundaries I Observations: Irregular extinction, complex grain boundaries Dynamic 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.

11. 0.1 mm 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: Recrystallisation at high T without deformation

12. New grown quartz can have idiomorphic crystal faces wherever pore space allows. 0.5 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).

13. 0,3 mm Chalcedony Nr. 29 Observations: "Brewster Crosses" indicating spherulithic fiber growth

14. Quartzine Long axis of indicatrix // fibers (Elongation): l' (+) Chalcedony ss Long axis of indicatrix at 90° to fibers (Elongation): l' (-) 0,3 mm Chalcedony, Disctinction between Quartzine and Chalcedony ss

15. K-Feldspar Formula : 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.

16. 1 mm Sanidine 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.

17. Microcline: horizontal Dispersion Optical properties of K-spars depend on Si-Al ordering in structure (010) (010) Intermediate Si-Ar ordering: pseudo uniaxial (010) (010) Nesse (2001) nach Steward & Ribbe (1983). Feldspar mineralogy, Rev. Mineral., MSA High-Sanidine: Dispersion at an angle However, such consocopic images are only seen in thick sanidine crystals specifiallcaly Nesse (2001) after Su et al. (1984). Am. Mineral. 69, 440-448

18. 1 mm Sanidine • 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

19. 0,3 mm Microcline, Domains of twins form typical crossed pattern Nr. 55 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

20. 0,3 mm Microcline, Myrmekite at Contact to Plagioclase Observation: Myrmekite are worm-shaped intergrowth patterns from replacement of K-feldspar by albite-rich plagioclase and quartz. Typical for granitic rocks.

21. 0,1 mm Microcline, perthitic Exsolutions Observation: Exsolutions always indicate difussive transport and thus slow cooling, e.g. in granitic rocks.

22. 0,3 mm Alkalifeldspar, Mesoperthite Observation: Plagioclase has higher refractive index than K-spar and thus plagioclase has a higher „relief“ (watch „Becke-Line“

23. 0,2 mm Alkalifeldspar, Fibrous Perthite

24. 0,05 mm Alkalifeldspar, Fibrous Perthite, Detai

25. 0,3 mm Antiperthite Observation: K-spar exsolutions in Ab-rich plagioclase

26. Plagioclase Formula : 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, volcanic Albite, plutonic Anorthite

27. (010) (010) Plagioclase, Albite twinning Twin plane : (010) Twin axis: (010) (010)

28. [001] Twin plane: (010) Twin axis: [010] (010) Plagioclase, Karlsbad-Twin

29. Plagioclase, How to recognise Albite twins : • Conditions to be met: • Both twins have identical interference colours when parallel N-S or E-W (i.e. parrallel to ploarizing planes). • At 45°- Position of lamella, twin plane no longer apparent (identical refractive index). • Both sets of twins have symmetrical extinction angle . a a

30. a a Your are not done yet, keep going Plagioclase, How to determine An - Ab composition I. after Rittmann (Maximum extinction angle against (010)), Part 1 • What to do ? • Find a crystal with orientation (010) ~ to plane of thin section by finding sharp traces of twin planes. • Verify albite twinning (see last slide). • Determine angle of extinction statistically: measure many, take average of maximum values found. • NOTE: Only the maximum angle represents the crystal with the correct orientation and will yield meaningful results. At 45°- positions twins can no longer be observed, twin planes blurred. Both sets of twins have identical interference colours when parallel to plane of ploarization (N-S, E-W).

31. Angle < 90° nx' Perikline twin nz' Angle >90° (001) (010) cleavage This Plagioclase hapens to be positive in the sense of the Michel-Levy rule. Note, this has nothing to do with the optical sign of the mineral! Keep going !! Plagioclase, How to determine An - Ab composition I. after Rittmann (Maximum extinction angle against (010)), Part 2 • (010) is the trace of the albite twin intergrowth, which you need to find. • (001) is paralel to the trace of cleavage or the trace of intergrowths or pericline twins. • Tue direction of maximum and minimum refraction is determined by the position with complete extinction. • At 45° nx' and nz' can be determined with the compensator (red I). 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)

32. The composition of this plagioclase is An67!! Plagioclase, How to determine An - Ab composition I. after Rittmann (Maximum extinction angle against (010)), Part 3 • 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) aus Tröger, 1971, p. 129

33. a1 a2 a1 a2 Plagioclase, How to determine An - Ab composition II. Method after Moorhouse (Extintion angle at Albit-Karlsbad twins) • Advantage: you need only one crystal, no statitics involved. • Disadvantage: approrpiate crystals with these comnbined albite-karlsbad twins rare and generally only in plutonic rocks. • What to do: • 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. • Determine the angle of extiction separately for both twins, giving two different values (unlike in the Rittman method). 2 1

34. Composition of the plagioclase is An70 !! Plagioclase, How to determine An - Ab composition II. Method after Moorhouse (Extintion angle at Albit-Karlsbad twins) • Value obtained here: • 40° large angle (twin ) • 18° small ange (twin 2) aus Moorehouse, 1959, p. 59

35. Composition of the plagioclase is An70 !! ...... 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 !!

36. ...... Crystals grow during cooling from a magma and thus their „growth-rings“ record what happened in the magma chamber prior to eruption !!