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Compact Course Microscopy of rock-forming Minerals PowerPoint PPT Presentation


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Compact Course Microscopy of rock-forming Minerals Part 7: Garnet, Andalusite, Sillimanite, Kyanite , Staurolite, Chloritoid, Corundum, Rutile, Spinel, Limonite, Opaque Ores, Artefacts, Miscellaneous. Garnet Formula:(Mg,Fe,Mn,Ca) 3 (Al,Fe,Cr) 2 [SiO 4 ] 3 Symmetriy: cubic n: 1,71 – 2,0

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Compact Course Microscopy of rock-forming Minerals

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Compact course 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


Compact course microscopy of rock forming minerals

Garnet

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.


Compact course microscopy of rock forming minerals

0,1 mm

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


Compact course microscopy of rock forming minerals

1 mm

Garnet

Observations:

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


Compact course microscopy of rock forming minerals

1 mm

Garnet

Observations:

Kelyphytic reaction rim around garnet.


Compact course microscopy of rock forming minerals

1 mm

Garnet

Observations:

.

  • Observations:

  • Deformation

  • Rotation of garnet

  • inclusion trails

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


Compact course microscopy of rock forming minerals

1 mm

Garnet

Observations:

.

  • Observations:

  • Deformation

  • Rotation of garnet

  • inclusion trails

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


Compact course microscopy of rock forming minerals

1 mm

Garnet

Observations:

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


Compact course microscopy of rock forming minerals

Andalusite

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.


Compact course microscopy of rock forming minerals

1 mm

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


Compact course microscopy of rock forming minerals

1 mm

Andalusite


Compact course microscopy of rock forming minerals

1 mm

Andalusite

Observations:

Large porphyroblasts with poikilitic texture.


Compact course microscopy of rock forming minerals

1 mm

Andalusite

Observations:

Chiastolite


Compact course microscopy of rock forming minerals

Sillimanite

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).


Compact course microscopy of rock forming minerals

1 mm

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


Compact course microscopy of rock forming minerals

0,5 mm

Sillimanite


Compact course microscopy of rock forming minerals

0,5 mm

Sillimanite, Fibrolite, orientiented inclusions in cordierite


Compact course microscopy of rock forming minerals

  • Observations:

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

  • oriented vertical to thin section plane (c vertical)

0,2 mm

Sillimanite, Fibrolite, Detail


Compact course microscopy of rock forming minerals

Kyanite

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.


Compact course microscopy of rock forming minerals

1 mm

Kyanite


Compact course microscopy of rock forming minerals

Trace of twin plane

1 mm

Kyanite

Observations:

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


Compact course microscopy of rock forming minerals

0,5 mm

Kyanite


Compact course microscopy of rock forming minerals

1 mm

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


Compact course microscopy of rock forming minerals

Staurolite

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.


Compact course microscopy of rock forming minerals

1 mm

Staurolite

Observations:

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


Compact course microscopy of rock forming minerals

1 mm

Staurolite

Nr. 63

Observations:

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


Compact course microscopy of rock forming minerals

1 mm

Staurolite

Nr. 57

Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.


Compact course microscopy of rock forming minerals

1 mm

Staurolite

Nr. 57

Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.


Compact course microscopy of rock forming minerals

0,5 mm

Staurolite

Nr. 38

Observations:

Andalusite-cystals with inclusions of staurolite.

Staurolite


Compact course microscopy of rock forming minerals

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.


Compact course microscopy of rock forming minerals

0,5 mm

Chloritoide

Observations:

Chloritoide porphyroblast overgrew older schistisity.


Compact course microscopy of rock forming minerals

Direction of polarizing plane

1 mm

Chloritoid

Observations:

Strong pleochrosim in Fe-rich chloritoide, polysynthetic twins.


Compact course microscopy of rock forming minerals

Corundum

Formula: 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}


Compact course microscopy of rock forming minerals

Observations:

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

1 mm

Corundum


Compact course microscopy of rock forming minerals

Observations:

Fractures parallel {1011}.

1 mm

Corundum


Compact course microscopy of rock forming minerals

0,5 mm

Corundum

  • Observations:

  • c vertical

  • c horizontal

  • with chloritoide and opaque ore


Compact course microscopy of rock forming minerals

Rutile

Formula: 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.


Compact course microscopy of rock forming minerals

0,5 mm

Rutile

  • Observations:

  • c vertical

  • cleavage at 90°!


Compact course microscopy of rock forming minerals

Spinel

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).


Compact course microscopy of rock forming minerals

1 mm

Spinel


Compact course microscopy of rock forming minerals

Limonite

Formula: FeOOH

Symmetry: rhombic

n: ~ 2 – 2,5

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.


Compact course microscopy of rock forming minerals

0,2 mm

Limonite


Compact course microscopy of rock forming minerals

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.


Compact course microscopy of rock forming minerals

to oculars and observer

illuminator

from light source

optical beam

splitter

sample

Reflected light microscope


Compact course microscopy of rock forming minerals

0,5 mm

Graphite

Observations:

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


Compact course microscopy of rock forming minerals

0,2 mm

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.


Compact course microscopy of rock forming minerals

1 mm

Magnetite

Observations:

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


Compact course microscopy of rock forming minerals

0,5 mm

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.


Compact course microscopy of rock forming minerals

Ilmenite

Magnetite

0,5 mm

Ilmenite

  • Observations:

  • elongated platy crystals: Ilmenite

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


Compact course microscopy of rock forming minerals

0,2 mm

Pyrite

Observations:

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


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