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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

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[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.


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


1 mm

Garnet

Observations:

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


1 mm

Garnet

Observations:

Kelyphytic reaction rim around garnet.


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.


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.


1 mm

Garnet

Observations:

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


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.


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


1 mm

Andalusite


1 mm

Andalusite

Observations:

Large porphyroblasts with poikilitic texture.


1 mm

Andalusite

Observations:

Chiastolite


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).


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


0,5 mm

Sillimanite


0,5 mm

Sillimanite, Fibrolite, orientiented inclusions in cordierite


  • 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


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.


1 mm

Kyanite


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.


0,5 mm

Kyanite


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


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.


1 mm

Staurolite

Observations:

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


1 mm

Staurolite

Nr. 63

Observations:

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


1 mm

Staurolite

Nr. 57

Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.


1 mm

Staurolite

Nr. 57

Observations:

Idiomorphic porphyroblasts growing over pre-existing schistosity texture.


0,5 mm

Staurolite

Nr. 38

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°

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.


0,5 mm

Chloritoide

Observations:

Chloritoide porphyroblast overgrew older schistisity.


Direction of polarizing plane

1 mm

Chloritoid

Observations:

Strong pleochrosim in Fe-rich chloritoide, polysynthetic twins.


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}


Observations:

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

1 mm

Corundum


Observations:

Fractures parallel {1011}.

1 mm

Corundum


0,5 mm

Corundum

  • Observations:

  • c vertical

  • c horizontal

  • with chloritoide and opaque ore


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.


0,5 mm

Rutile

  • Observations:

  • c vertical

  • cleavage at 90°!


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).


1 mm

Spinel


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.


0,2 mm

Limonite


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.


to oculars and observer

illuminator

from light source

optical beam

splitter

sample

Reflected light microscope


0,5 mm

Graphite

Observations:

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


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.


1 mm

Magnetite

Observations:

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


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.


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)


0,2 mm

Pyrite

Observations:

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


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