Sillimanite, an aluminum silicate mineral, was found in the gneiss at Sterling Hill. It has not been reported from the orebodies or the Franklin Marble. It was found as colorless 1-3 mm crystals, associated with quartz and a garnet in the almandine-pyrope series on the 1500 level, in the central core of gneiss at Sterling Hill.
The sillimanite used to establish the granulite facies degree of regional metamorphism was found on Hamburgh Mountain to the east of the Franklin-Sterling Hill area. (Dunn, 1995)

 Location Found: Ogdensburg
 Year Discovered: 1824
 Formula: Al2(SiO4)O
 Essential Elements: Aluminum, Oxygen, Silicon
 All Elements in Formula: Aluminum, Oxygen, Silicon
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
 To find out more about this mineral at minDat's website, follow this link   Sillimanite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.382

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.76

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