Sillimanite
Sillimanite, Al2SiO5: orthorhombic, hardness 6.5 to 7.5, density 3.23 to 3.24. Kyanite and andalusite share its formula, and fibrolite is a fibrous variety.
Sillimanite is aluminium silicate, Al2SiO5, an IMA species (G, 1824, USA): orthorhombic, hardness 6.5 to 7.5, density 3.23 to 3.24, with one perfect {010} cleavage, prismatic to fibrous. Andalusite and kyanite share its composition. A 20.00 g piece reads 13.81 to 13.83 g in water; kyanite reads at least 0.51 g heavier.
In short
- Three rows, one composition. A row parser over the September 2026 IMA-CNMNC Master List, checked by a field scan, finds exactly 3 rows for the aluminium silicate: andalusite, Al2SiO5, G, 1798, Spain; sillimanite, Al2SiO5, G, 1824, USA; and kyanite, written Al2OSiO4, A, 1967 s.p., Austria. Each Handbook of Mineralogy sheet prints Al2SiO5 and calls the species trimorphous with the other two.
- The half-gram rule. Worked as 20 − 20/D on the sheets’ measured densities, a 20.00 g piece reads 13.81 to 13.83 g in water as sillimanite, 13.61 to 13.67 g as andalusite and 14.33 to 14.52 g as kyanite. Kyanite is at least 0.51 g heavier than either of the other two, enough for a balance reading to 0.01 g.
- Documented failure: sillimanite against andalusite. The sheets leave only 0.14 g between them. The International Gem Society gives compact sillimanite a specific gravity of 3.14 to 3.18, which reads 13.63 to 13.71 g, overlapping andalusite’s 13.61 to 13.67 g. Weight cannot split compact sillimanite from andalusite; the cleavage and the refractive indices can.
- The refractometer separates all three on the sheet ranges. Andalusite’s indices run 1.632 to 1.643, sillimanite’s 1.653 to 1.683 and kyanite’s 1.710 to 1.734, with no overlap. That is an instrument test, not a home one.
- Fibrolite and where. The gem society calls fibrolite the fibrous variety and names greyish-green chatoyant fibrolite from Sri Lanka. The Handbook’s Distribution field names the Lisens Alp, Tirol; Maršová, Czech Republic; Bodenmais, Bavaria; Chester and Norwich, Connecticut; gem crystals from the Okkampitiya, Ratnapura, Deniyaya and Balangoda districts, Sri Lanka; and Christmas Point, Antarctica.
| Criterion | Sillimanite | Andalusite | Kyanite | Separates sillimanite? |
|---|---|---|---|---|
| IMA row | Al2SiO5; G, 1824, USA | Al2SiO5; G, 1798, Spain | Al2OSiO4; A, 1967 s.p., Austria | No: same composition |
| Crystal system | Orthorhombic, Pbnm | Orthorhombic, Pnnm | Triclinic, P-1 | Only on a crystal |
| Habit | Prismatic to acicular, square cross sections, to 2 cm; fibrous mats | Euhedral crystals or columns with nearly square cross sections, to 20 cm; also fibrous | Bladed, tabular on {100}, to 0.5 m; typically bent or twisted | Partly: both orthorhombic ones can be fibrous |
| Cleavage | Perfect on {010} | Good on {110}, poor on {100} | Perfect on {100}, good on {010}, at 79° | Yes, on a broken edge |
| Hardness | 6.5 to 7.5 | 6.5 to 7.5 | 5.5 along [001], 7 along [100] | Not from andalusite |
| Density, measured | 3.23 to 3.24 | 3.13 to 3.16 | 3.53 to 3.65 | From kyanite only |
| 20.00 g piece in water | 13.81 to 13.83 g | 13.61 to 13.67 g | 14.33 to 14.52 g | From kyanite: at least 0.51 g |
| Refractive indices | α 1.653 to 1.661, γ 1.672 to 1.683 | α 1.632, γ 1.643 | α 1.710 to 1.718, γ 1.724 to 1.734 | Yes, on an instrument |
| Optic sign | Biaxial (+) | Biaxial (−) or (+) | Biaxial (−) | From kyanite |
| Colour (sheet) | Colourless, white to gray, brown, yellow, green, blue | Pink, violet, yellow, green, white, gray | Blue, white, rarely green, gray, yellow, pink, black | Partly |
| Occurrence (sheet) | High-grade metamorphic schists, gneisses, hornfels | Contact metamorphism of argillaceous sediments; regional schists | Moderately high-pressure regional metamorphism of pelitic rocks | A clue, not a test |
What is sillimanite? The fibrous aluminium silicate
Sillimanite is orthorhombic aluminium silicate, Al2SiO5. The Handbook of Mineralogy sillimanite sheet gives point group 2/m 2/m 2/m and space group Pbnm, with crystals prismatic, may be acicular, with square cross sections, rounded and striated, poorly terminated, to 2 cm, and as fibrous mats, rarely radiating. Cleavage is perfect on {010}; the fracture is uneven and aggregates are tough. Hardness is 6.5 to 7.5 and measured density 3.23 to 3.24.
Colour and optics. The sheet gives colourless or white to gray, also brown, yellow, yellow-green, gray-green, blue-green and blue, with a white streak and a vitreous to subadamantine lustre. It is biaxial (+), α 1.653 to 1.661, β 1.657 to 1.662, γ 1.672 to 1.683, 2V 21° to 30°, and coloured material is pleochroic, X pale brown or pale yellow, Y brown or green, Z dark brown or blue.
Its chemistry, worked. On standard atomic weights Al2SiO5 is 62.92% Al2O3 and 37.08% SiO2, the figures the sheet prints for the ideal. Its analysis from Delaware Co., Pennsylvania, SiO2 37.08, Al2O3 63.11, Fe2O3 0.09, recalculates on five oxygens to Al 2.00, Si 1.00, as the sheet says. The IMA list gives sillimanite status G, 1824, USA, with a first reference in the American Journal of Science and Arts 8 (1824), 113; the sheet marks no type locality. The name honours Benjamin Silliman of Yale.
Sillimanite, andalusite and kyanite: one formula, three species
Sillimanite, andalusite and kyanite have the same composition and different structures. Each of the three Handbook sheets prints Al2SiO5 and says the species is trimorphous with the other two. A row parser over the September 2026 IMA list, checked by an independent scan of every formula field, finds exactly 3 rows for the composition: andalusite and sillimanite written Al2SiO5, and kyanite written Al2OSiO4, which states the same atoms with one oxygen set apart from the SiO4 group. Kyanite is also the only one of the three with an A status, 1967 s.p.; the other two are grandfathered.
What each sheet says about where it forms. The sillimanite sheet: in high-grade metamorphic schists, gneisses and hornfels, more rarely in pegmatites, also detrital. The andalusite sheet: a result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists. The kyanite sheet: from moderately high-pressure regional metamorphism of principally pelitic rocks. Each lists the other two among its associates. This page quotes the sheets and draws no pressure-temperature diagram; it has no fetched source for one.
Kyanite’s own page. The two-direction hardness test that identifies kyanite is set out there and not repeated here; the sheet numbers on this page agree with its kyanite figures. For the two orthorhombic species, the andalusite sheet gives euhedral crystals or columnar aggregates with nearly square cross sections to 20 cm, good cleavage on {110}, poor on {100}, density 3.13 to 3.16, and pink, violet, yellow, green, white or gray colour.
Can you tell sillimanite from andalusite and kyanite by weight? The half-gram rule
Weight separates kyanite from the other two, and fails between sillimanite and andalusite. Worked as 20 − 20/D on the measured densities, a 20.00 g piece reads 13.81 to 13.83 g in water as sillimanite, 13.61 to 13.67 g as andalusite and 14.33 to 14.52 g as kyanite. Kyanite’s lowest reading is 0.51 g above sillimanite’s highest and 0.66 g above andalusite’s. Quartz, for scale, reads 12.45 g. The method is on how to measure specific gravity at home.
Documented failure. On the sheet figures sillimanite and andalusite are 0.14 g apart, with the midpoint at 13.74 g. The International Gem Society sillimanite article gives sillimanite a specific gravity of 3.23 to 3.27, compact varieties 3.14 to 3.18, and Kenyan crystals 3.27. Compact sillimanite at 3.14 to 3.18 reads 13.63 to 13.71 g, inside andalusite’s window. A reading near 13.7 g therefore names neither.
What does separate them. Sillimanite has one perfect cleavage, on {010}; andalusite’s is good on {110} and poor on {100}; kyanite’s is perfect on {100} and good on {010}, the two meeting at 79°. Reading a broken edge is on cleavage or fracture. On a refractometer the sheet ranges do not overlap: andalusite 1.632 to 1.643, sillimanite 1.653 to 1.683, kyanite 1.710 to 1.734.
What is fibrolite? Sillimanite’s fibrous variety
Fibrolite is sillimanite: the International Gem Society treats it as the fibrous variety. The International Gem Society sillimanite article heads its entry sillimanite (=fibrolite) and says the name fibrolite is in allusion to the fibrous nature of this variety. It names green, blue and violet-blue facetable material from Sri Lanka and Burma, greyish-green chatoyant fibrolite from Sri Lanka, and pale bluish to colourless facetable crystals from Kenya. The Handbook sheet does not use the word, but records sillimanite as fibrous mats, and a case-insensitive scan of the IMA list finds 0 lines containing fibrolite.
Andalusite has its own variety names. The International Gem Society andalusite article describes chiastolite as gray crystals with black, carbonaceous inclusions in cruciform patterns, and notes the type locality question: the material first described came from Castile-La Mancha, and it gives El Cardoso de la Sierra, Guadalajara as the type locality, where the Handbook sheet says the name is for an occurrence in the Andalusia region. The same article calls viridine a grass to deep green variety containing manganese. Neither chiastolite nor viridine has a line on the IMA list.
Where sillimanite is found. The Distribution field opens of widespread occurrence, but fine examples are uncommon, and names the Lisens Alp, Selraintal, Tirol, Austria; Maršová (Marschendorf), Czech Republic; Bodenmais, Bavaria, Germany; Pontgibaud, Auvergne, France; Chester, near Saybrook, and near Norwich, Connecticut, Chester, Delaware Co., Pennsylvania, and Monroe, Orange Co., New York; gem crystals from the Okkampitiya, Ratnapura, Deniyaya and Balangoda districts, Sri Lanka; and Christmas Point, Enderby Land, Antarctica. Its associates are andalusite, kyanite, potassic feldspar, almandine, cordierite, biotite and quartz; the kyanite sheet adds staurolite and corundum to its own associates. The reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.