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

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: 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.

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).

Compact Course Microscopy of rock-forming Minerals