source of course material dr. olaf medenbach, ruhr-universit ä t bochum optical microscopy of...

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

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.

GarnetObservations:

Kelyphytic reaction rim around garnet.

GarnetObservations:

Observations:

• Deformation

• Rotation of garnet

• inclusion trails

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

GarnetObservations:

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.

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

Andalusite

Observations:

Large porphyroblasts with poikilitic texture.

Andalusite

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

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

Observations:

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

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

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

Observations:

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

Staurolite Nr. 63

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.

Staurolite Nr. 57

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

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.

CorundumObservations:

Fractures parallel {1011}.

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

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

LimoniteFormula : FeOOH

Symmetry : rhombic

n : ~ 2 – 2,5

n : 0,6 0,6 (L.), 0,08-0,14(G.)

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

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).

source of course material dr. olaf medenbach, ruhr-universit ä t bochum optical microscopy of...

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