Sillimanite

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SILLIMANITE – Al2SiO5 
 Sillimanite 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 Color Habit Cleavage 
Ortorhombic 
bipiramidal. 
Generally colorless, 
may be gray, white, 
yellow, blue, green or 
brown. 
Almost always long 
prismatic to fibrous. 
Square basal sections. 
{010} perfect. 
Tenacity 
Brittle. 
Twinning Fracture Mohs Hardness Parting 
No. Irregular. 6.5 – 7.5 No. 
Streak Lustre Diaphaneity Density (g/cm3) 
White. Subvitreous, silky. Transparent. 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 
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. 
CPL 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. 
CL 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(-). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Sillimanite, diagonally across the image, in long prismatic crystals in PPL (left, colorless) and and in CPL
(right, blue, red and orange colors). Typical are the prismatic habit, fractures perpendicular to the elongation, 
the relief from medium to high and the intense colors in CPL. Generally, as here, the crystals are long and 
thin and the cleavage is not visible. 
 
 
In CPL, diagonally (SW-NE) 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. 
 Basal sections of sillimanite, typically square to rectangular with diagonal cleavage, in PPL (left) and in CPL 
(right). Below them a short sillimanite in longitudinal section. In PPL, the high relief is very conspicuous. In 
CPL, 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. 
 
 
In the center of the images, diagonally, a prismatic sillimanite crystal in longitudinal section. Around it, 
chloritoid (PPL: green-blue), magnetite (opaque) and cordierite (CPL: gray). In PPL (left), sillimanite is 
colorless; its high relief in this paragenesis is difficult to perceive. In CPL (right), sillimanite has strong colors. 
It resembles muscovite, but muscovite shows mottled extinction and much lower relief. 
 
In the center of the images, diagonally, prismatic sillimanite crystal in longitudinal section, with opaques and 
cordierite, in PPL (left) and in CPL (right). In this case, it is similar to the association “muscovite+quartz”. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Sillimanite, variety fibrolite, in PPL (light cream-
colored fibrous masses in the center of the image). 
 
Details of the fibrolite variety of sillimanite in PPL (left) and in CPL (right). They are fibrous masses, forming 
irregular clusters that present both the intense interference colors of sillimanite and gray colors, depending on 
the section. Image taken with the medium magnification (10x) and zoom of the camera. 
 
The two images make it possible to compare the relief of muscovite (lamellae at the top, roughly in the 
center, diagonally, PPL: colorless, CPL: green-yellow colors) with the much higher relief of sillimanite (PPL: 
colorless, CPL: orange -red, below the muscovite lamellae). In gray, cordierite. 
 
Sillimanite in PPL, with blue and red 
interference colors, forming an aggregate of 
prismatic crystals tending to radial. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Left: inPPL, with 40x magnification objective, glaucophane (blue), sillimanite (long prisms, colorless, medium 
relief), garnet (rounded grains, colorless, high relief) and quartz (colorless, low relief). Thanks to their higher 
relief, sillimanite prisms stand out. 
Right: in CPL, sillimanite prisms show their typical strong colors. They resemble muscovite, but do not have 
mottled extinction. 
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. 
 
PPL Reflection color: Light gray, approximately the same color as quartz and feldspars, much 
lighter than micas. 
Pleochroism: No. 
Reflectivity: Low (