1. Microscopy of rock-forming Minerals
Compact Course
Microscopy of rock-forming Minerals
Part 7: Garnet, Andalusite, Sillimanite, Kyanite, Staurolite, Chloritoid, Corundum, Rutile, Spinel, Limonite, Opaque Ores, Artefacts, Miscellaneous

2. Garnet Observations: Formula :(Mg,Fe,Mn,Ca)3(Al,Fe,Cr)2[SiO4]3
Symmetriy : cubic
n : 1,71 – 2,0
n : -
2V : -
max. I. F. (30μm) : -
Observations:
Isotropic, high relief, often idiomorphic, colour rose to brownish-red. Can only be confuses with spinel. Difference are in shape and chagrins.
Spinel crystallized in octahedra (shapes with 3, 4 or max. 6 corners. Garnet shows rounded shapes.

3. Garnet
Observations:
idiomorophic garnet crystals next to amphibole, included in quartz.
Note: very small crystals can appear to be non-isotropic due to the underlying quartz.
perfect rhombic-
dodekaeder
0,1 mm

4. Garnet
Observations:
Poicilitic growth of garnet, including many other crystals gives a sieve-like texture.
1 mm

5. Garnet
Observations:
Kelyphytic reaction rim around garnet.
1 mm

6. Garnet Observations: Deformation Rotation of garnet inclusion trails
Such rotated garnets are called: „snow ball garnets“. They grow during the deformation of the rock.
Observations: .
1 mm

7. Garnet Observations: Deformation Rotation of garnet inclusion trails
Such rotated garnets are called: „snow ball garnets“. They grow during the deformation of the rock.
Observations: .
1 mm

8. Garnet
Observations:
Atoll-shaped garnet due to partial replacement of a particular growth zone, which may be instable for its distinct compositin.
1 mm

9. Andalusite Observations: Formula : Al[5]Al[6][O|SiO4]
Symmetriy : orthorhombic
n : 1,629 – 1,640
n : 0,009 – 0,011
2Vx : 83° - 85°
max. I. F. (30μm) : white I. order
Observations:
Generally prismatic, ceavage following {110} at 90°, sometimes pleochroic. Chiastolite with inclusions forming a typical 90° cross. In metamorphic rocks at relatively low P and T, contact metamorphic slates.

10. Andalusite Observations:
Compositional zonation in andalusite with rose-to clolourless pleochroism in the core. Note claevage following {110} at 90° in sections with c
c vertical
c horizontal
1 mm

11. Andalusite
1 mm

12. Andalusite Observations: Large porphyroblasts with poikilitic texture.
1 mm

13. Andalusite
Observations:
Chiastolite
1 mm

14. Sillimanite Observations: Formula : Al[4]Al[6][O|SiO4]
Symmetry : orthorhombic
n : 1,657 – 1,682
n : 0,02 – 0,022
2Vz : 21° - 30°
max. I. F. (30μm) : white I. order
Observations:
Typically long-prismatic to fibrous crystals. Larger crystals show one direction of cleavage{010}
Optical character in elongation is positive (important because small crystals will not give interference images of axes).

15. Sillimanite Observations:
Zoned Sillimanite with rose-colourless pleochroism in the core. Note cleavage along {110} at 90° in sections with vertical c.
c vertical
c horizontal
1 mm

16. Sillimanite
0,5 mm

17. Sillimanite, Fibrolite, orientiented inclusions in cordierite
0,5 mm

18. Sillimanite, Fibrolite, Detail
Observations:
oriented parallel to thin section plane (elongated sections, c horizontal)
oriented vertical to thin section plane (c vertical)
0,2 mm

19. Kyanite Observations: Formula : Al[6]Al[6][O|SiO4]
Symmetry : triclinic
n : 1,710 – 1,729
n : 0,016 – 0,017
2Vz : 82°
max. I. F. (30μm) : Orange I. order
Observations:
Due to perfect cleavage and rigid mechanical behaviour crystals in thin sections often show break-outs.

20. Kyanite
1 mm

21. Kyanite Observations:
Twin with both crystals almost in identical optical orientation, note the slight angle between cleavage planes of the twins.
Trace of twin plane
1 mm

22. Kyanite
0,5 mm

23. Kyanite Observations:
There are three main components, all with high refractive index. Try to distinguish..:
Garnet: isotropic character
Jadeite: retrograde reaction rims
Kyanite: no reaction rims
1 mm

24. Staurolite Observations: Formula : 2FeO•AlOOH•4Al2[O|SiO4]
Symmetry : monoclinic
n : 1,739 – 1,762
n : 0,013 – 0,015
2Vz : 79° - 90°
max. I. F. (30μm) : Orange I. order
Observations:
High refractive index, low birefringence, yellow to brown colour, twins often observed.

25. Staurolite Observations:
Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.
1 mm

26. Staurolite Nr. 63 Observations:
Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.
1 mm

27. Staurolite Nr. 57 Observations:
Idiomorphic porphyroblasts growing over pre-existing schistosity texture.
1 mm

28. Staurolite Nr. 57 Observations:
Idiomorphic porphyroblasts growing over pre-existing schistosity texture.
1 mm

29. Staurolite Nr. 38 Observations:
Andalusite-cystals with inclusions of staurolite.
Staurolite
0,5 mm

30. Chloritoide Observations: Formula : (Fe,Mg,Mn)Al2[(OH)2|O|SiO4]
Symmetry : monoclinic and triclinic
n : 1,682 – 1,740
n : 0,010 – 0,012
2Vz : 36° - 70°
max. I. F. (30μm) : white I. order
Observations:
High refractive index, low birefringence, blue-green colour, twins often observed. Anomalous blueish interference colours near position of extinction.

31. Chloritoide Observations:
Chloritoide porphyroblast overgrew older schistisity.
0,5 mm

32. Direction of polarizing plane
Chloritoid
Observations:
Strong pleochrosim in Fe-rich chloritoide, polysynthetic twins.
Direction of polarizing plane
1 mm

33. Corundum Observations: Formula : Al2O3 Symmetry : trigonal
max. I. F. (30μm) : white I.order
Observations:
Easy to recognize: high refractive index similar to garnets, low birefringence similar to quartz, uniaxial.
Rose to blue-coloured.
Typical fractures parallel {1011}

34. Corundum Observations:
Typical fractures parallel{1011}are not a cleavage ! as indicated by effects of anisotropy on these planes.
1 mm

35. Corundum
Observations:
Fractures parallel {1011}.
1 mm

36. Corundum Observations: c vertical c horizontal
with chloritoide and opaque ore
0,5 mm

37. Rutile Observations: Formula : TiO2 Symmetry : tetragonal
max. I. F. (30μm) : white I. order
Observations:
Mineral with the highes refractive index and highest birefringence. Always intesively yellow to brown colours, weak pleochroism ne>no. Interference image of axis difficult to see due to high chagrins. Cleavage {110} (90°). Twins.

38. Rutile
Observations:
c vertical
cleavage at 90°!
0,5 mm

39. Spinel Observations: Formula : (Fe,Mg,Mn)(Al,Fe3+,Cr)2O4
Symmetry : cubic
n : 1,718 – 2,05
n : -
2V : -
max. I. F. (30μm) : -
Observations:
Isotropic, high relief, very similar to garnets. Spinels, however, crystallize in octahedral shape and thus cyrstals have few faces (3, 4 to max. faces).

40. Spinel
1 mm

41. Limonite Observations: Formula : FeOOH Symmetry : rhombic
n : 0,6 (L.), 0,08-0,14(G.)
2V :
max. I. F. (30μm) : very high, white
Observations:
Mixture of goethite and lepidokrokit, often rhythmically intergrown. Results from alteration and oxidation of Fe-bearing sulphides (pyrite, chalkopyrite, etc.). „Rust“ that is a finely dispered brown pigment in weathered rocks.

42. Limonite
0,2 mm

43. (Fe-Ti oxides, sulphides, graphite)
Opaque minerals
(Fe-Ti oxides, sulphides, graphite)
To investigate opaque minerals, reflected light microscopes are needed.
Only in rare cases, from crystal shapes and alteration products, some information on this group of minerals can be gained even with a transmitted light microscope. Examples follow. .
Research microscope with transmitted and reflected light.

44. to oculars and observer
Reflected light microscope
to oculars and observer
optical beam
splitter
sample
illuminator
from light source

45. Graphite Observations:
Elongated plates, often deformed and bent and kinked, splayed margins like staked paper (similar to micas)
0,5 mm

46. Magnetite Observations:
With ilmenite most abundant Fe-Ti Oxide and opaque minerals in igneous rocks. Crystal shape : octahedra -> isometric cross sections, mostle triangular to cubic, more rarels hexagonal sections.
0,2 mm

47. Magnetite Observations:
Isometriis cross sectionss, examples of fast-frown crystal with sceletal shape.
1 mm

48. Chromite - Magnetite Observations:
Zoned crystal from solid solution series between magnesiochromite (MgCr2O4) and chromite (FeCr2O4). Magnesiochromite is brown translucent. Typical for mafic and ultramafic igneous rocks.
0,5 mm

49. Ilmenite Observations: elongated platy crystals: Ilmenite
6-sided cross sections : ilmentige with c vertical of a cubic opaque mineral (magnetite)
Ilmenite
Magnetite
0,5 mm

50. Pyrite
Observations:
Fe-sulphide altered to limonite at the rim (pyrite or chalkopyrite).
0,2 mm