SILLIMANITE


Al2(SiO4)O,
aluminum silicate




Within the formal Franklin-Sterling Hill area, sillimanite to date has been found only in the central gneiss zone on 1500 level of the Sterling mine, where it occurs as tiny, hair-thin, colorless fibers in a light-colored, quartz-almandine-feldspar gneiss with saccharoidal (sugary) texture. It has not been found elsewhere within either orebody, or within the host Franklin Marble (Dunn, 1995, p. 382), but is known from gneissic rocks in several localities nearby. Sterling Hill sillimanite is nearly invisible to the naked eye and generally requires close scrutiny with a hand lens to detect. It is most common, and certainly most readily visible, within the pink, garnet-rich patches of the host gneiss (SIL6b), but even there it is generally sparse, most such patches hosting no more than one or two fibers of the mineral. The fibers are generally short, averaging 2 to 4 mm; the longest measured to date is 7.5 mm.

Sillimanite is one of the more unsung of the local minerals, probably due in part to its underwhelming physical appearance and tiny size of its crystals. To geologists, however, it is an important indicator mineral, for it occurs only in aluminous rocks that have been subjected to high temperatures of metamorphism. The minerals andalusite and kyanite, both unknown locally, have the same composition as sillimanite but form under different conditions of temperature and pressure. Because all three minerals are common on a worldwide basis, maps of their presence or absence within any large area of metamorphic rock enable geologists to recognize broad regional differences in degrees of metamorphism. The high metamorphic grade of the local Proterozoic rocks, as discussed by Hague et al. (1956), was inferred in part from the widespread presence of sillimanite in aluminous rocks of the Franklin-Sterling Hill area.

Local sillimanite, as with nearly all sillimanite worldwide, has not been reported to fluoresce under ultraviolet light.

It should be noted that few mine passages at Sterling Hill intersected the central gneiss zone, thus explaining, perhaps in part, the scarcity of sillimanite from that locality. The photos below show sillimanite not only from Sterling Hill, but also, for purposes of comparison, from several other areas in aluminous gneissic rocks nearby. The most notable of these is about 2.6 km southeast of Ogdensburg, on Hamburg Mountain, where sillimanite occurs in quartz-microcline-biotite gneisses of the Edison mines area (see map by Volkert and Monteverde, 2013). Specimens SIL1, SIL7, and SIL9 are from that area. Remarkable spindle-shaped crystals of sillimanite have been collected at a now-lost locality north of Franklin (SIL2, SIL3), and smaller ones (SIL4) near Rudetown, also north of Franklin.

References

Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey:  the world’s most magnificent mineral deposits. Privately published in five volumes, 755 pp. [Short description of sillimanite on p. 382]

Hague, J.M., Baum, J.L., Herrmann, L.A., and Pickering, R.J. (1956), Geology and structure of the Franklin-Sterling area, New Jersey. Geological Society of America Bulletin, vol. 67, no. 4, p. 435-474, with map. [Remarks on common presence of sillimanite within aluminous rocks of the area as an indicator of high temperatures of metamorphism]

King, V.T., King, N.E., Betancourt, P.P., Bostwick, R.C., Chin, Peter, Hecht, T.J., Kuitems, S.M., Moritz, Harold, Nemetz, J.D., Nikischer, A.J., Sanford, Stephen, Van Fleet, J.A., and Verbeek, E.R. (2022), The Mineralogy of Franklin and Ogdensburg, New Jersey—A Photographic Celebration. Privately published, 1400 pp. [Several photographs of Sterling mine sillimanite shown on p. 1078]

Verbeek, E.R. (2019), Sillimanite from Sussex County, New Jersey. The Picking Table, vol. 60, no. 1, p. 29-31. [Description and photos of sillimanite from several nearby localities]

Volkert, R.A. and Monteverde, D.H. (2013), Bedrock geologic map of the Franklin quadrangle, Sussex and Morris Counties, New Jersey. New Jersey Geological and Water Survey, Geologic Map Series map 13-3, scale 1:24,000.



Images




Tiny (1-3 mm), slender, prismatic sillimanite crystals, grains of pink almandine in a pale gray, quartz-oligoclase-almandine gneiss
Earl R. Verbeek
The sillimanite in this specimen, from Sterling Hill, occurs as tiny (1-3 mm), slender, prismatic crystals within grains of pink almandine in a pale gray, quartz-oligoclase-almandine gneiss with saccharoidal texture. Small scales of a brown mica are present throughout but are widely scattered. The sillimanite is neither abundant nor conspicuous but is obvious upon hand-lens inspection of the garnet grains (see photo SIL6b). This specimen, FMM-1964 of the Franklin Mineral Museum, at one time was in the collection of Ewald Gerstmann. The Gerstmann collection was later purchased by Art and Harriet Mitteldorf, who in 1985 donated it to the Franklin Mineral Museum.

Identifier: SIL6a
Locality: 1500 level, central gneiss zone, Sterling mine, Ogdensburg
Specimen size: 14.5 x 11 x 6.5 cm



Tiny (1-3 mm), slender, prismatic sillimanite crystals, grains of pink almandine in a pale gray, quartz-oligoclase-almandine gneiss
Vandall T. King
A photomicrograph of needlelike crystals of sillimanite within the gneiss specimen shown in photo SIL6a. In addition to the sillimanite crystal in the center of the image, note the spray of smaller crystals farther up, with their right ends embedded in pink garnet and their left ends embedded in quartz.

Field of view: The horizontal width is 2 mm.

Identifier: SIL6b
Locality: 1500 level, central gneiss zone, Sterling mine, Ogdensburg
Specimen size: 14.5 x 11 x 6.5 cm





Abundant, pale greenish yellow, bladed, tapering crystals of sillimanite in feldspar-quartz-magnetite gneiss with minor apatite
Richard Schnurr (Fox Tail Images)
Abundant, pale greenish yellow, bladed, tapering crystals of sillimanite, most 1-2.5 cm long and in rough alignment, in feldspar-quartz-magnetite gneiss with minor apatite. This example is from the collection of Joe Orosz, former curator of the Franklin Mineral Museum, and almost certainly was collected by him during one of his visits to the Edison mines. It was donated to the FMM by his widow, Sharon Orosz, in late 2017, and assigned catalog number FMM-7988. As noted by Dunn (1995), sillimanite from this area was “used to establish the granulite facies degree of regional metamorphism” of the local Proterozoic rock units.

Identifier: SIL1b
Locality: Edison mines, about 2.6 km southeast of Ogdensburg, in Sparta Township
Specimen size: 9 x 7.5 x 3 cm





Small white crystals of sillimanite in biotite matrix in a specimen of biotite-feldspar-quartz gneiss
Elaine Whetstine
A profusion of small white crystals of sillimanite in biotite matrix in a specimen of biotite-feldspar-quartz gneiss from the Edison mines. Like specimen SIL1b, this specimen likely was collected by Joe Orosz and was donated to the Franklin Mineral Museum by his widow, Sharon, in late 2017. It now resides in the FMM collection as specimen FMM-7929.

Identifier: SIL7a
Locality: Edison mines, about 2.6 km southeast of Ogdensburg, in Sparta Township
Specimen size: 15 x 8 x 4 cm





Slender, white, needlelike crystals of sillimanite with biotite in a biotite-sillimanite-feldspar-magnetite-garnet gneiss
Elaine Whetstine
Slender, white, needlelike crystals of sillimanite with biotite in a biotite-sillimanite-feldspar-magnetite-garnet gneiss. Note the large porphyroblasts of feldspar in this specimen and, on left side as photographed, the grains of red garnet. This is Franklin Mineral Museum specimen FMM-7957 and, like others from this locality, came from the Joe Orosz collection via donation from Sharon Orosz.

Identifier: SIL9a
Locality: Edison mines, about 2.6 km southeast of Ogdensburg, in Sparta Township
Specimen size: 10.5 x 6.5 x 3 cm





Long (8-10 cm), tapering, spindle-shaped crystals of pale tan sillimanite in a microcline-biotite-quartz gneiss matrix
Earl R. Verbeek
Long (8-10 cm), tapering, spindle-shaped crystals of pale tan sillimanite in parallel alignment, lying flat within a foliation plane in a microcline-biotite-quartz gneiss matrix. This specimen was collected by John L. Baum, first curator of the Franklin Mineral Museum, during his exploration field work for the New Jersey Zinc Company. Subsequently it became part of the worldwide mineral collection of Wilfred and Mary Welsh, who in 1991 donated their entire collection to the FMM. It now bears catalog number FMM-2207. See also related specimen SIL3a.

Identifier: SIL2a
Locality: Owens, north of present-day McAfee, in northern Vernon Township, along the western base of Pochuck Mountain, near the intersection of Owens Station Road (County Route 642) and Lake Wallkill Road. The exact locality is unknown, but this area is about 16 km north-northeast of Franklin.
Specimen size: 16 x 14 x 2.5 cm





Prismatic crystals of pale tan sillimanite, most 7-12 mm in diameter, projecting from the surface of a fine-grained microcline-biotite-quartz gneiss
Earl R. Verbeek
Prismatic crystals of pale tan sillimanite, most 7-12 mm in diameter, projecting from the surface of a fine-grained microcline-biotite-quartz gneiss. The sillimanite crystals in this specimen (FMM-2206) are viewed "head on," and only short segments of them are visible. The crystals are mutually parallel and define a lineation in the host gneiss. In a companion specimen from the same locality (SIL2a) the crystals are seen to be spindle-shaped and 8-10 cm long.

Identifier: SIL3a
Locality: Owens, north of present-day McAfee, in northern Vernon Township, along the western base of Pochuck Mountain, near the intersection of Owens Station Road (County Route 642) and Lake Wallkill Road. The exact locality is unknown, but this area is about 16 km north-northeast of Franklin.
Specimen size: 17 x 8.5 x 3 cm





Pale yellow, long-bladed crystals of sillimanite in hornblende-quartz-feldspar gneiss
Earl R. Verbeek
Pale yellow, long-bladed crystals of sillimanite in slightly divergent groups, lying flat on a foliation plane in hornblende-quartz-feldspar gneiss. This specimen, FMM-2208 of the Franklin Mineral Museum, was collected by John L. Baum on October 8, 1989, during his exploration field work for the New Jersey Zinc Company and subsequently donated to the FMM.

Identifier: SIL4a
Locality: Rudetown, Sussex Co., NJ. The locality is about 5-6 km northeast of Franklin, between present-day Hardistonville and McAfee.
Specimen size: 8 x 2.5 x 2 cm