Mineral · Al2SiO5 polymorphs
Sillimanite
Al2(SiO4)O
Sillimanite is the high-temperature Al2SiO5 polymorph, usually fibrous (fibrolite) in gneiss and schist, with rare gem crystals from Sri Lanka and Myanmar and cat's-eye material from India. It is Delaware's state mineral. Collectors hold it mainly to complete the kyanite–andalusite–sillimanite trio.
| Formula | Al2(SiO4)O |
|---|---|
| IMA status | grandfathered |
| Crystal system | Orthorhombic |
| Mohs hardness | 6.5–7.5 |
| Group | Al2SiO5 polymorphs |
| Type locality | Chester, Connecticut, USA |
| Streak | White |
| Specific gravity | 3.24 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Elsewhere
Cite this article
Stratum Strata. "Sillimanite" (Mineral). Stratum Strata pack 2026.09, as of 2026-09-21. https://stratum.black/strata/minerals/sillimanite. Accessed 2026-09-21.
@misc{stratum-sillimanite,
title = {Sillimanite},
author = {{Stratum Strata}},
howpublished = {\url{https://stratum.black/strata/minerals/sillimanite}},
year = {2026},
note = {Mineral article, Stratum Strata pack 2026.09, as of 2026-09-21; sources: stratum, wikidata, commons. Accessed 2026-09-21.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.