What does sillimanite mean?
Sillimanite is a mineral composed of aluminum silicate, with the formula Al₂SiO₅. It is commonly found as fibrous, prismatic, or needle-like crystals in high-grade metamorphic rocks and is especially associated with the regional metamorphism of aluminum-rich sediments. Its presence can help geologists infer the temperatures and pressures that affected a rock. Sillimanite shares its chemical composition with andalusite and kyanite, but each has a different crystal structure and develops under distinct metamorphic conditions. For a general reader, the word is useful because it illustrates how identical chemical ingredients can form different minerals when subjected to changing geological environments. It is primarily a technical term in geology and mineralogy rather than everyday vocabulary.
nounA mineral with the chemical formula Al₂SiO₅ that occurs in high-temperature, aluminum-rich metamorphic rocks. Sillimanite is one of three minerals with this composition, the others being andalusite and kyanite.
- The orthorhombic aluminum-silicate mineral Al₂SiO₅, often occurring as fibrous or needle-like crystals in high-grade metamorphic rocks.
"The metamorphic rock contains fibrous sillimanite crystals."
"Geologists identified sillimanite in the schist as an indicator of high-temperature metamorphism."
"The museum displays a polished specimen of fibrous sillimanite."
The plural is uncommon and may refer to separate mineral specimens or distinct occurrences; the mineral is also frequently used as an uncountable mass noun.
"The collection includes several fine sillimanites from Alpine metamorphic rocks."
Sillimanite belongs to a remarkable trio: andalusite, kyanite, and sillimanite share the same chemistry yet can be formed from the same material under different pressures and temperatures.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does sillimanite come from?
The name honors Benjamin Silliman, the American mineralogist and chemist who contributed to the study of minerals and their chemical composition. The word entered English in the 19th century in scientific usage and retains the surname in the form “Silliman”.
How sillimanite is actually used
The word is used in geology and mineralogy and is uncommon in general usage. It is normally treated as a mass noun when referring to the mineral, although the plural “sillimanites” can be used for distinct specimens or occurrences.
Easily confused with sillimanite
Sillimanite and andalusite have the same chemical formula but differ in crystal structure and typically form under different metamorphic conditions.
Kyanite has the same chemical composition as sillimanite but is usually denser, forms different crystals, and is associated with higher-pressure metamorphism.