Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Al2SiO5 Most sillimanite is relatively pure, although minor amounts of Fe3+, Cr3+, Ti4+ may be present.
Crystal System orthorhombic a=7.488, b=7.681, c=5.777.
Crystal Habit Slender prismatic or fine fibrous crystals. Masses of fine fibrous crystals, known as fibrolites, may form swirled or matted aggregates. Prismatic crystals crystals may appear rectangular or diamond shaped in cross section with diagonal cleavage lines.

Perfect on (010)

Uneven fracture. Brittle. Fibrous aggregates may be tough.
Color/Pleochroism Colorless or white, less commonly yellow, brown or blue. Vitreous luster and white streak.  
Optic Sign Biaxial (+)   
2V 20-30


Optic Orientation X=a

- Optic axis dispersion strong, r > v.
- Fragments on (010) cleavage yield flash figures.

Refractive Indices
alpha = 
beta = 
gamma = 
delta = 

Max Birefringence 0.018-0.022 Indices of refraction show little variation. Interference colors in thin section, are up to lower second order. Fine fibrolite fibers show lower inerference colors because they are hinner than a standard thin section.
Elongation length slow  
Extinction parallel   
Distinguishing Features High relief, moderate birefringence, parallel extiction, and slender prismatic or fibrous crystal habit. Distinctive diamond-shaped cross sections with cleavage for prismatic grains. Fibrous mats commonly replace biotite.
Occurrence Sillimanite occurs as a constituent of high-temperature metamorphosed rocks containing clay. In contact metamorphosed rocks it may occur in silimanite-cordierite gneisses or sillimanite-biotite horfels. In silica poor rocks, it may be associated with corundum.
Editors Mai Kobayashi ('06), Lyn Watts ('17)


Crossed polarized light photomicrograph of silliminite whowing one principle textural type: needle-like fibrolite from a quartz-muscovite-silliminite-biotite schist.

Plain polarized and cross polarized light of fibrous sillimanite It is elongated and needle-like, and can be seen to be surrounding small amounts of muscovite.
Plain polarized and cross polarized images of prismatic sillimanite. The diamond shapes are diagnostic with visible cleavage.
WWW University of Minnesota
Union College [under amphiboles]
Oxford University [images 17-21]

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