source of course material dr. olaf medenbach, ruhr-universit ä t bochum optical microscopy of...
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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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
GarnetObservations:
Poicilitic growth of garnet, including many other crystals gives a sieve-like texture.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
GarnetObservations:
Kelyphytic reaction rim around garnet.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
GarnetObservations:
Atoll-shaped garnet due to partial replacement of a particular growth zone, which may be instable for its distinct compositin.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Andalusite
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Andalusite
1 mm
Observations:
Large porphyroblasts with poikilitic texture.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Andalusite
1 mm
Observations:
Chiastolite
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
SillimaniteFormula : 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).
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Sillimanite
0,5 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Sillimanite, Fibrolite, orientiented inclusions in cordierite
0,5 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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)
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Kyanite
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Kyanite
0,5 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Staurolite
1 mm
Observations:
Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Staurolite Nr. 63
1 mm
Observations:
Idiomorphic, poikilitic crystals as porphyroblasts in a contact metamorphic shist, together with large crystals of andalusite.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Staurolite Nr. 57Observations:
Idiomorphic porphyroblasts growing over pre-existing schistosity texture.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Staurolite Nr. 57
1 mm
Observations:
Idiomorphic porphyroblasts growing over pre-existing schistosity texture.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Staurolite Nr. 38
0,5 mm
Observations:
Andalusite-cystals with inclusions of staurolite.
Staurolite
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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°
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Chloritoide
0,5 mm
Observations:
Chloritoide porphyroblast overgrew older schistisity.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Chloritoid
1 mm
Observations:
Strong pleochrosim in Fe-rich chloritoide, polysynthetic twins.
Direction of polarizing planeDirection of polarizing plane
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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}
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
CorundumObservations:
Typical fractures parallel{1011}are not a cleavage ! as indicated by effects of anisotropy on these planes.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
CorundumObservations:
Fractures parallel {1011}.
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Corundum
Observations:
• c vertical
• c horizontal
with chloritoide and opaque ore
0,5 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
RutileFormula : TiO2
Symmetry : tetragonal
n : 2,6 – 2,9
n : 0,3 !!!!!
2V : -
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Rutile
Observations:
• c vertical
• cleavage at 90°!
0,5 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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).
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Spinel
1 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Limonite
0,2 mm
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Reflected light microscope
to oculars and to oculars and observerobserver
from light from light sourcesource
optical beam optical beam splittersplitter
samplesample
illuminatorilluminator
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Graphite
0,5 mm
Observations:
Elongated plates, often deformed and bent and kinked, splayed margins like staked paper (similar to micas)
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Magnetite
1 mm
Observations:
Isometriis cross sectionss, examples of fast-frown crystal with sceletal shape.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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.
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
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
Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum
Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei
Pyrite
0,2 mm
Observations:
Fe-sulphide altered to limonite at the rim (pyrite or chalkopyrite).