SILLIMANITE

Sillimanite – Al2SiO5 – is a rarer nesosilicate, typical of medium to high grade metamorphic rocks, used as an industrial mineral in the production of ceramics.

It is one of the polymorphs of the Al2SiO5 Group, together with kyanite and andalusite. It is usually very pure; but may contain some Cr and Fe. The fibrous form is called fibrolite; it is usually white to yellowish and can occur in parallel or twisted, rarely radial fiber bundles, involving crystals of neighboring minerals.

1. Characteristics

Crystal system:  Ortorhombic bipiramidal.         

Color: Generally colorless, may be gray, white, yellow, blue, green or brown.     

Habit: Almost always long prismatic to fibrous. Square basal sections.       

Cleavage: {010} perfect.       

Tenacity: Brittle.        

Twinning: No.       

Fracture:  Irregular.      

Mohs Hardness: 6.5 – 7.5

Parting: No.         

Streak: White.         

Lustre:  Subvitreous, silky.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.23 – 3.27

          

2. Geology and Deposits

Sillimanite is a high-pressure, high-temperature Al2SiO5 Group polymorph. It occurs in pelitic metamorphic rocks with high Al content, of medium to high grade, such as mica-schists, gneisses (sillimanite-cordierite-gneisses) and cornubianites (sillimanite-biotite-cornubianites).

It is also found in granitic rocks, where it is a common accessory mineral. Occurs in pegmatites. Due to its high hardness, sillimanite can be detrital, occurring in placers and sedimentary rocks.

 

3. Mineral Associations

Sillimanite occurs associated with quartz, K-feldspar, kyanite, andalusite, garnet (almandine), staurolite, prismatine, mica (muscovite and biotite), cordierite and pyrite.

In rocks with low silica content, it can be associated with corundum.

When two or even three Al2SiO5 polymorphs occur in the same thin section, usually only one is thermodynamically stable. Typically, a rock, during an orogenesis-related metamorphism, passes from the kyanite stability field to that of sillimanite. As the reaction is slow, kyanite may still be present in metastable form.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1.653 – 1.661    nβ: 1.657 – 1.662       nγ: 1.672 – 1.683

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, usually without pleochroism. In thick (>30nm) thin sections it can be brownish, yellowish or bluish, in this case it presents weak to moderate pleochroism.

Fibrolite (acicular sillimanite) can be dirty yellowish-brown.  

Relief: Moderate to high.           

Cleavage: {010} perfect, only visible in larger (wider) crystals, where the cleavage runs parallel to the elongation. In the basal sections, which are pseudotetragonal (square to rectangular), the cleavage is arranged diagonally.           

Habits: Typically long prismatic, almost acicular, with fractures perpendicular to the elongation, forming radiated aggregates. Wider prisms show cleavage parallel to the elongation. The sections of these prisms are square (pseudotetragonal), have 1st order gray to white colors, diagonal cleavage (diagnostic!) and an interference figure of excellent quality (perpendicular to the acute bisector). It can be fibrous (then called fibrolite), constituting coiled, twisted, mat-like aggregates.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.018 to 0.022, resulting in superior 1st order to 2nd order interference colors: yellow, red, blue, greenish.

Colors do not appear on needle-like crystals.

Fibrolite fibers have lower colors because they are thinner than the thickness of a thin section (30 nm).           

Extinction: Parallel to the cleavage. In the basal sections it is symmetric.           

Elongation sign: ES(+) (diagnostic!) Distinguishes, associated with the interference colors, the sillimanite from corundum and beryl, which have very similar optical properties and occur in the same paragenesis, but have lower interference colors (gray) and SE(-). 

Twins: No.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 21-30º, normally cannot be measured.         

Alterations: hydrating, it alters to micas such as muscovite (sericitization) and biotite. With P and T changes, several changes to other minerals are possible.          

May be confused with: the long prismatic to fibrous habit, associated with high relief, parallel extinction, positive Elongation Sign and moderate birefringence are very diagnostic.

Apatite and andalusite have ES(-).

Kyanite has oblique extinction, is B(-), shows lower interference colors, and has 2 perfect cleavages.

Zoisite has lower birefringence.

Thicker prismatic crystals can be confused with tremolite, but tremolite has oblique extinction and occurs in other paragenesis.

The fibrolite variety bears some resemblance to fibrous amphibole and fibrous andalusite, but these usually have strong colors in PPL and/or distinct pleochroism.

Dumortierite in some cases is pink with pleochroism, resembles fibrolite, but has SE(-).         

 

In XPL, diagonally in the image, in strong yellow, red and blue colors, long prismatic crystals of sillimanite in metamorphic rock. Relief, colors, habit, parallel extinction, ES(+) and paragenesis are very characteristic.
Sillimanite 01: Sillimanite, diagonally across the image, in long prismatic crystals in PPL. ). Typical are the prismatic habit, fractures perpendicular to the elongation, the relief from medium to high. Aorund it, biotite, quartz and others.
Sillimanite 01: The same section as of the previous image, now in XPL. Sillimanite with blue/red interference colors. Generally, as here, the crystals are long and thin and the cleavage is not visible.
Sillimanite 02: Basal sections of sillimanite, typically square to rectangular with diagonal cleavage, in PPL. Below them a short sillimanite in longitudinal section. High relief is very conspicuous.
Sillimanite 02: The same section as of the previous image, now in XPL. The colors of the basal sections vary between gray and black; the prismatic section displays very strong colors.
Pseudotetragonal basal sections of sillimanite in PPL. They are colorless sections with medium to high relief that tend to be square and with diagonal cleavage, a very diagnostic feature, as there is no other common mineral that presents this feature.
Pseudotetragonal basal sections of sillimanite in PPL. They are colorless sections with medium to high relief that tend to be square and with diagonal cleavage, a very diagnostic feature, as there is no other common mineral that presents this feature.
Sillimanite 03: In PPL, in the center of the image, diagonally, a prismatic sillimanite crystal in longitudinal section. Around it, chloritoid (PPL: green-blue), magnetite (opaque) and cordierite (PPL: colorless). Sillimanite is colorless; its high relief in this paragenesis is difficult to perceive.
Sillimanite 03: The same section as of the previous image, now in XPL. Sillimanite has strong colors. It resembles muscovite, but muscovite shows mottled extinction and much lower relief.
Sillimanite 04: In the center of the image, diagonally, prismatic sillimanite crystal in longitudinal section, with opaques and cordierite.
Sillimanite 04: The same section as of the previous image, now in XPL. Sillimanite in blue/red interference colors.
Sillimanite, variety fibrolite, in PPL (light cream-colored fibrous masses in the center of the image).
Sillimanite 05: Fibrolite variety seen in detail. They are fibrous masses, forming irregular clusters that present both the intense interference colors of sillimanite and gray colors, depending on the section.
Sillimanite 05: The same section as of the previous image, now in XPL. Strong interference colors or gray can both be seen.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of sillimanite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with sillimanite, like magnetite and pyrite.

Sample preparation: because of its excellent cleavage, silimanite polish is never perfect, especially on larger (wider) crystals. There will always be holes, which do not have the triangular shapes of classic polishing pits. The number of these holes depends on the orientation of the cleavage in relation to the plane of the polished section. But generally sillimanite crystals polish quite well, just as easily as common silicates such as quartz and feldspar.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, approximately the same color as quartz and feldspars, much lighter than micas. 

Pleochroism: No.      

Reflectivity: Low (<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Does not show anisotropy.        

Internal reflections: Generalized in light colors, very often multicolored due to the cleavage.      

May be confused with: other light-colored transparent minerals, but the long prismatic shape, excellent cleavage, and the resulting colorless and multicolored reflections are very diagnostic, evidently considering paragenesis. Tremolite may be similar, but occur in other paragenesis.

Sillimanite 01: Diagonally across the image, a long prismatic sillimanite crystals. Around it, micas (darker) and other silicates (quartz, etc.). The form is very typical for sillimanite.
Sillimanite 01: The same section as of the previous image, now in XPL. The internal reflections can be just light/whitish or multicolored as in this image. The crystal on the right, almost vertical, has large (black) holes generated by the orientation of its cleavage in relation to the plane of the polished section.
Sillimanite 02: In PPL, beam of prismatic crystals of sillimanite. As they are very fine crystals and with very good cleavage, there are many holes (polishing pits) (black in PPL).
Sillimanite 02: The same section as of the previous image, now in XPL. Multicolored reflections in some of the crystals are typical.
In PPL, horizontally, two beams of long prismatic crystals, almost acicular, of sillmanite. In white grey, pyrrhotite. In dark brownish grey, micas. In light grey, granular, other silicates.