Source of course material Dr. Olaf Medenbach, Ruhr-Universit ä t Bochum Optical microscopy of rock-forming minerals, G. W ö rner, 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 7: Garnet, Andalusite, Sillimanite, Kyanite, Staurolite, Chloritoid, Corundum, Rutile, Spinel, Limonite, Opaque Ores, Artefacts, Miscellaneous



GarnetFormula :(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.



Garnet

0,1 mm

Observations:

idiomorophic garnet crystals next to amphibole, included in quartz.

NoteNote: : very small crystals can appear to be non-isotropic due to the underlying quartz.

perfect rhombic-dodekaeder



GarnetObservations:

Poicilitic growth of garnet, including many other crystals gives a sieve-like texture.

1 mm



GarnetObservations:

Kelyphytic reaction rim around garnet.

1 mm



GarnetObservations:

.

1 mm

Observations:

• Deformation

• Rotation of garnet

• inclusion trails

• Such rotated garnets are called: „snow ball garnets“. They grow during the deformation of the rock.



GarnetObservations:

.

1 mm

Observations:

• Deformation

• Rotation of garnet

• inclusion trails

• Such rotated garnets are called: „snow ball garnets“. They grow during the deformation of the rock.



GarnetObservations:

Atoll-shaped garnet due to partial replacement of a particular growth zone, which may be instable for its distinct compositin.

1 mm



AndalusiteFormula : 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.



AndalusiteObservations:

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



Andalusite

1 mm



Andalusite

1 mm

Observations:

Large porphyroblasts with poikilitic texture.



Andalusite

1 mm

Observations:

Chiastolite



SillimaniteFormula : Al[4]Al[6][O|SiO4]

Symmetry : orthorhombic

n : 1,657 – 1,682

n : 0,02 – 0,022

2Vz : 21° - 30°


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).



SillimaniteObservations:

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



Sillimanite

0,5 mm



Sillimanite, Fibrolite, orientiented inclusions in cordierite

0,5 mm



Sillimanite, Fibrolite, Detail

0,2 mm

Observations:

• oriented parallel to thin section plane (elongated sections, c horizontal)

• oriented vertical to thin section plane (c vertical)



KyaniteFormula : 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.



Kyanite

1 mm



Kyanite

Observations:

Twin with both crystals almost in identical optical orientation, note the slight angle between cleavage planes of the twins.

1 mm

Trace of twin plane

Trace of twin plane



Kyanite

0,5 mm



KyaniteObservations:

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



StauroliteFormula : 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.



Staurolite

1 mm

Observations:

Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.



Staurolite Nr. 63

1 mm

Observations:

Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.



Staurolite Nr. 57Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.

1 mm



Staurolite Nr. 57

1 mm

Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.



Staurolite Nr. 38

0,5 mm

Observations:

Andalusite-cystals with inclusions of staurolite.

Staurolite



Chloritoide 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°


Observations:

High refractive index, low birefringence, blue-green colour, twins often observed. Anomalous blueish interference colours near position of extinction.



Chloritoide

0,5 mm

Observations:

Chloritoide porphyroblast overgrew older schistisity.



Chloritoid

1 mm

Observations:

Strong pleochrosim in Fe-rich chloritoide, polysynthetic twins.

Direction of polarizing planeDirection of polarizing plane



CorundumFormula : Al2O3

Symmetry : trigonal

n : 1,759 – 1,772

n : 0,008 – 0,009

2V : -

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}



CorundumObservations:

Typical fractures parallel{1011}are not a cleavage ! as indicated by effects of anisotropy on these planes.

1 mm



CorundumObservations:

Fractures parallel {1011}.

1 mm



Corundum

Observations:

• c vertical

• c horizontal

with chloritoide and opaque ore

0,5 mm



RutileFormula : TiO2

Symmetry : tetragonal

n : 2,6 – 2,9

n : 0,3 !!!!!

2V : -


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.



Rutile

Observations:

• c vertical

• cleavage at 90°!

0,5 mm



SpinelFormula : (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).



Spinel

1 mm



LimoniteFormula : FeOOH

Symmetry : rhombic

n : ~ 2 – 2,5

n : 0,6 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.



Limonite

0,2 mm



Research microscope with transmitted and reflected light.

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.

.



Reflected light microscope

to oculars and to oculars and observerobserver

from light from light sourcesource

optical beam optical beam splittersplitter

samplesample

illuminatorilluminator



Graphite

0,5 mm

Observations:

Elongated plates, often deformed and bent and kinked, splayed margins like staked paper (similar to micas)



Magnetite

0,2 mm

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.



Magnetite

1 mm

Observations:

Isometriis cross sectionss, examples of fast-frown crystal with sceletal shape.



Chromite - Magnetite

0,5 mm

Observations:

Zoned crystal from solid solution series between magnesiochromite (MgCr2O4) and chromite (FeCr2O4). Magnesiochromite is brown translucent. Typical for mafic and ultramafic igneous rocks.



Ilmenite

0,5 mm

Observations:

• elongated platy crystals: Ilmenite

• 6-sided cross sections : ilmentige with c vertical of a cubic opaque mineral (magnetite)

MagnetiteMagnetite

IlmeniteIlmenite



Pyrite

0,2 mm

Observations:

Fe-sulphide altered to limonite at the rim (pyrite or chalkopyrite).

Documents

Source of course material Dr. Olaf Medenbach, Ruhr-Universit ä t Bochum Optical microscopy of rock-forming minerals, G. W ö rner, USTC Hefei Compact Course